Engineered extracellular matrices control stem cell behavior

ABSTRACT

A composition for culturing stem cells is provided. The composition comprises an engineered purified collagen based matrix that has been formed under controlled conditions to have the desired microstructure and mechanical properties. The engineered purified collagen based matrix compositions of the present invention can be used alone or in combination with cells as a tissue graft construct to enhance the repair of damaged or diseased tissues

FIELD OF THE INVENTION

This invention relates to the preparation of a collagen based matrix forculturing and differentiating stem cells and progenitor cells and theuse of such compositions as tissue graft constructs.

BACKGROUND

The interaction of cells with their extracellular matrix (ECM) as itoccurs in vivo plays a crucial role in the organization, homeostasis,and function of tissues and organs. Continuous communication betweencells and their surrounding ECM environment orchestrates criticalprocesses such as the acquisition and maintenance of differentiatedphenotypes during embryogenesis, the development of form(morphogenesis), angiogenesis, wound healing, and even tumor metastasis.Both biochemical and biophysical signals from the ECM modulatefundamental cellular activities including adhesion, migration,proliferation, differential gene expression, and programmed cell death.

In turn, the cell can modify its ECM environment by modulating thesynthesis and degradation of specific matrix components. The realizationof the significance of cell-ECM interaction has led to a renewedinterest in characterizing ECM constituents and the basic mechanisms ofcell-ECM interaction.

Tissue culture allows the study in vitro of animal cell behavior in aninvestigator-controlled physiochemical environment. Presumably culturedcells function best (i.e., proliferate and perform their natural in vivofunctions) when cultured on substrates that closely mimic their naturalenvironment. Currently, studies in vitro of cellular function arelimited by the availability of cell growth substrates that present theappropriate physiological environment for proliferation and developmentof the cultured cells. Complex scaffolds representing combinations ofECM components in a natural or processed form are commerciallyavailable, such as Human Extracellular Matrix (Becton Dickinson) andMATRIGEL®. However, none of the existing scaffolds have been preparedunder conditions that regulate the polymerization of the scaffold in acontrolled manner so as to produce a composition having mechanicalproperties and a predetermined 3D microstructure of collagen fibrilsand/or soluble ECM components that optimizes cell-substrate interactionsto yield predictable and reproducible cellular outcomes. Applicants havediscovered that the physical state of an ECM scaffold and not just itsmolecular composition should be considered in the design of new andimproved scaffolds.

As reported herein, modifying the conditions used to form a collagenbased matrix from a solubilized collagen solution allows for thecontrolled alteration of the micro-structural and subsequent mechanicalproperties of the resulting ECM scaffold. Furthermore, themicro-structural and mechanical properties of the ECM scaffold directlyimpact fundamental cell behavior including survival, adhesion,proliferation, migration and differentiation of cells cultured withinthe scaffold.

Basement membrane tissues and submucosal material harvested from warmblooded vertebrates have shown great promise as unique graft materialsfor inducing the repair of damaged or diseased tissues in vivo, and forsupporting fundamental cell behavior (e.g., cell proliferation, growth,maturation, differentiation, migration, adhesion, gene expression,apoptosis and other cell behaviors) of cell populations in vitro.Submucosal material can be extracted or fluidized to provide enrichedextracts that can be utilized as additives for tissue culture media, orpolymerized to form collagen based scaffolds, to promote in vitro cellgrowth and proliferation.

As a tissue graft, submucosal tissue undergoes remodeling and inducesthe growth of endogenous tissues upon implantation into a host. Numerousstudies have shown that submucosal tissue is capable of inducing hosttissue proliferation, remodeling and regeneration of tissue structuresfollowing implantation in a number of in vivo environments, includingthe urinary tract, the body wall, tendons, ligaments, bone,cardiovascular tissues and other vascular tissues, and the centralnervous system. Upon implantation of the submucosal tissues, cellularinfiltration and a rapid neovascularization are observed and thesubmucosa materials are remodeled into host replacement tissue withsite-specific structural and functional properties.

Accordingly, submucosa tissue can be used as a tissue graft construct,for example, in its native form, in its fluidized form, in the form ofan extract, or as components extracted from submucosa tissue andsubsequently purified. The fluidized forms of vertebrate submucosatissue can be gelled to form a semi-solid composition that can beimplanted as a tissue graft construct or utilized as a cell culturesubstrate. As a tissue graft material, the fluidized form can beinjected, or delivered using other methods, to living tissues to enhancetissue remodeling. Furthermore, the fluidized form can be modified, orcan be combined with specific proteins, growth factors, drugs, plasmids,vectors, or other therapeutic agents for controlling the enhancement oftissue remodeling at the site of injection. Moreover, the fluidized,solubilized form can be combined with primary cells or cell lines priorto injection to further enhance the remodeling properties that result inthe repair or replacement of diseased or damaged tissues.

Because the molecular forces that orchestrate the self assembly ofsoluble, monomeric collagen into higher ordered structures are weaktheir assembly can easily turn into an unstructured aggregation ofmisfolded proteins. In the literature, there are known methods forisolating collagen from a variety of tissues, e.g., placenta and animaltails and using the isolated material to reconstitute collagenousmatrices. These known methods rely on the protein's intrinsic ability toretain its secondary structure during protein isolation and assume that,for instance, the alpha helix will retain its helical structurethroughout. The end result, even with a homogenous biochemicalcomposition, can be a heterogeneous secondary structure. Controlling theassembly of the constituting monomers into tertiary or quaternarymultimeric arrangements is very hard to achieve under such conditions.One embodiment of the present invention is directed to controlling thepolymerization of a composition comprising solubilized collagen to forma collagen based scaffold that has the requisite microstructure andcomposition to allow for the expansion, differentiation and/or clonalisolation of stem cells in a highly reproducible and predictable manner.

SUMMARY

The present invention relates to compositions comprising a threedimensional matrix that is formed to have the requisite composition andmicrostructure to enhance the proliferation and/or differentiation ofstem cells or progenitor cells cultured within such a matrix. Inaccordance with one embodiment an improved method for culturing stemcells is provided. The method comprises preparing a solubilized collagencomposition from a source of collagen, adding cells to the solubilizedcollagen composition and polymerizing the collagen composition undercontrolled conditions to provide a matrix formed from collagen fibrilsand having the desired microstructure. In one embodiment cells are addedto the collagen based matrix at a cell density within two orders ofmagnitude of the minimum cell number required to maintain cellviability, and the cells are cultured under conditions suitable forproliferation of the cells. In one embodiment the three dimensionalmatrix has a fibril area fraction (defined as the percent area of thetotal area occupied by fibrils in a cross-sectional surface of thematrix; providing an estimate of fibril density) of about 8% to about26% and an elastic or linear modulus (defined by the slope of the linearregion of the stress-strain curve) of about 0.5 to about 40 kPa. In oneembodiment the three dimensional matrix is further provided with anexogenous source of glucose and calcium chloride.

In accordance with one embodiment, stem cell seeded engineered purifiedcollagen based matrices are used as novel compositions for inducing therepair of damaged or disease tissues in vivo. In one embodiment thetissue graft construct comprises an engineered purified collagen basedmatrix, wherein the matrix is formed by contacting purified collagenwith hydrochloric acid to produce a solubilized collagen composition andsubsequently polymerizing the solubilized collagen composition undercontrolled conditions and in the presence of a population of cells toproduce the engineered purified collagen based matrix containing cellsentrapped within the matrix. In one embodiment the population of cellscomprises stem cells initially added to the composition at a density ofless than 10⁵ cells per milliliter, or the progeny of such stem cells.In one embodiment the stem cell seeded engineered purified collagenbased matrices are implanted into a host without culturing the seededstem cells in vitro. In another embodiment the stem cell seededengineered purified collagen based matrix is further incubated underconditions suitable for inducing the proliferation and/ordifferentiation of the seeded stem cells.

In another embodiment the stem cells are added to the engineeredpurified collagen based matrices at densities of less than 10³ cells permilliliter and the cells are cultured under conditions that areminimally permissive for stem cell functionality. These conditionsresult in the production of localized populations of stem cells and thusallow for the isolation of clonal populations of stem cells.Accordingly, in one embodiment, a method of isolating clonal populationsof individual stem cells is provided. The method comprises the steps ofcontacting a collagen based matrix with a low density of stem cellswherein said collagen matrix is formed by contacting a source ofcollagen with HCl to prepare a solubilized collagen composition,polymerizing the solubilized collagen composition using a final collagenconcentration of 1.0 to 3.0 mg/ml, at a pH of about 6.5 to about 7.0. Inone embodiment the initial seeded population of stem cells ranges fromabout 10 to about 10³ cells per milliliter. The seeded stem cells arecultured under conditions suitable for proliferation of the cells andindividual populations of stem cells are isolated.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A-1F present data showing the effect of various parameters on thestiffness (elastic or linear modulus) of the formed matrix. FIG. 1Arepresents the effect of polymerization temperature on a matrix formedfrom a solubilized collagen composition comprising 1 mg/ml collagen in1×PBS at pH 7.4; FIG. 1B represents the effect of the buffer type on amatrix formed from a solubilized collagen composition comprising 1 mg/mlcollagen, and about 0.15 M NaCl at 37° C.; FIG. 1C represents the effectof pH (using a phosphate buffer) on a matrix formed from a solubilizedcollagen composition comprising 1 mg/ml collagen, in 1×PBS at pH 7.4;FIG. 1D represents the effect of pH (using a tris buffer) on a matrixformed from a solubilized collagen composition comprising 1 mg/mlcollagen, in 50 mM tris, and about 0.15 M NaCl at 37° C. FIG. 1Erepresents the effect of ionic strength on a matrix formed from asolubilized collagen composition comprising 1 mg/ml collagen, no buffer,at 37° C.; FIG. 1F represents the effect of phosphate concentration on amatrix formed from a solubilized collagen composition comprising 1 mg/mlcollagen, and about 0.15 M NaCl at 37° C.; FIG. 1G represents the effectof SIS component concentration on a matrix formed from a solubilized ECMcollagen composition in 1×PBS at 37° C.

FIGS. 2A & 2B represent a series of graphs showing the quantification offibril area fraction (FIG. 2A) and fibril diameter distribution (FIG.2B) based upon confocal and SEM images, respectively. All fibril areafraction relationships showing statistically significant differences(p<0.05) are indicated with symbols (*, **, , ◯).

FIGS. 3A-3D represent a series of graphs showing cell length (FIG. 3A),length/width ratio (FIG. 3B), width (FIG. 3C), and surface area (FIG.3D) determined and compared for neonatal human dermal fibroblasts(NHDFs) seeded within 3D ECMs prepared with 1.5 mg/ml type I collagen,and a type III collagen content that varied from 0 to 0.75 mg/ml.Results represent the means and standard deviations for 10≦n≦23 cellsanalyzed for each ECM formulation at a given time point. All groupsshowing statistically significant differences (p<0.05) are marked withthe same symbol.

FIGS. 4A-4D represent a series of images depicting cell contractilityand matrix remodeling by individual NHDFs resident within type Icollagen (1.5 mg/ml) ECMs prepared with type III collagen concentrationsof 0.25 mg/ml (FIGS. 4A and 4B) and 0.75 mg/ml (FIGS. 4C and 4D). FIGS.4A and 4C represent 2D projections of confocal reflection image stacksshowing changes to NHDF morphology and collagen fibril microstructureobserved 5 hours after polymerization. FIGS. 4B and 4D representquantified levels of local volumetric strain (matrix deformation) withinthe 3D tissue construct.

FIG. 5 represents a graph depicting contractility and matrix remodelingwithin engineered ECMs. NHDFs were grown within engineered ECMs in whichthe type I collagen concentration was kept constant at 1.5 mg/ml and theamount of type III collagen was either 0.25 mg/ml or 0.75 mg/ml. Averagelocal 3D principal strains for a single cell and its surrounding ECMwere quantified 5 hours post-polymerization (5≦n≦6). Negative strainvalues indicate compressive deformations. All relationships showingstatistically significant differences (p<0.05) are indicated withsymbols (*, **, , ◯, □, +).

FIG. 6 represents a graph showing that points of maximum localdeformation or strain induced within a 3D tissue construct, by lowpassage neonatal human dermal fibroblasts, occurred at distances furtherfrom the cell than for engineered ECMS prepared with lower amounts oftype III collagen. NHDFs were grown within engineered ECMs in which thetype I collagen concentration was kept constant at 1.5 mg/ml and theamount of type III collagen was either 0.25 mg/ml or 0.75 mg/ml.

FIGS. 7A & 7B represent a series of graphs depicting data regarding theproliferation of low passage human dermal fibroblasts when grown withina 3D ECM format consisting of type I collagen ECMs prepared withinincreasing amounts of type III collagen (see FIG. 7A). NHDFs exposed to2D ECM surface coatings representing the same biochemical compositionsand collagen type I/III ratios showed no significant changes inproliferative response (see FIG. 7B). Selected relationships showingstatistically significant differences (p<0.05) are indicated withsymbols (*, **, , ◯).

FIG. 8 represents a bar chart showing differences in the expression ofselect tissue-specific genes by multi-potential bone marrow derivedmesenchymal cells grown on standard 2D plastic and within 3D ECMmicroenvironments of increased fibril density and stiffness (elastic orlinear modulus). Gene expression patterns for mesenchymal cells culturedwithin a given 2D or 3D format was also modulated by changing thecomposition of the culture medium.

FIG. 9 is a schematic representation of general cell behavior ofmulti-potential bone marrow derived mesenchymal cells when culturedwithin 3D matrices that differ in collagen concentration to provide anECM microenvironment characterized by increased fibril density andstiffness (elastic or linear modulus). Points of arrow indicate lowfrequency events and wide ends of arrows indicate high frequency events.

DETAILED DESCRIPTION Definitions

As used herein, the term “stem cell” refers to an unspecialized cellfrom an embryo, fetus, or adult that is capable of self-replication orself-renewal and can develop into specialized cell types of a variety oftissues and organs. The term as used herein, unless further specified,encompasses totipotent cells (those cells having the capacity todifferentiate into extra-embryonic membranes and tissues, the embryo,and all post-embryonic tissues and organs), pluripotent cells (thosecells that can differentiate into cells derived from any of the threegerm layers), and multipotent cells (those cells having the capacity todifferentiate into a limited range of differentiated cell types).

As used herein the term “progenitor cell” refers to a stem cell withmore specialization and less differentiation potential than a totipotentstem cell. For example, progenitor cells include unipotential cells(those cells having the capacity to differentiate along a single celllineage).

As used herein, the term “lyophilized” relates to the removal of waterfrom a composition, typically by freeze-drying under a vacuum. However,lyophilization can be performed by any method known to the skilledartisan and the method is not limited to freeze-drying under a vacuum.Typically, the lyophilized tissue is lyophilized to dryness, and in oneembodiment the water content of the lyophilized tissue is belowdetectable levels.

As used herein “solubilized collagen composition” refers to acomposition that comprises collagen in a predominantly soluble monomericform (for example wherein less than 20% of the collagen is insoluble,denatured, or assembled in higher ordered structures).

As used herein “solubilized extracellular matrix composition” refers toa naturally occurring extracellular matrix that has been treated, forexample, with an acid to reduce the molecular weight of at least some ofthe components of the extracellular matrix and to produce a compositionwherein at least some of the components of the extracellular matrix havebeen solubilized from the extracellular matrix. The “solubilizedextracellular matrix composition” may include insoluble components ofthe extracellular matrix as well as solubilized components.

As used herein the term “collagen-based matrix” refers to extracellularmatrices that comprise collagen. An “engineered purified collagen basedmatrix” as used herein relates to a composition comprising a collagenfibril scaffold that has been formed under controlled conditions from asolubilized collagen composition, wherein the solubilized collagencomposition is prepared from a composition consisting essentially ofcollagen. The conditions controlled during the polymerization reactioninclude one or more of the following: pH, phosphate concentration,temperature, buffer composition, ionic strength, and composition andconcentration of purified collagen components. Similarly, an “engineeredextracellular matrix” relates to a solubilized extracellular matrixcomposition that is polymerized to form a collagen fibril containingmatrix under controlled conditions, wherein the controlled conditionsinclude pH, phosphate concentration, temperature, buffer composition,ionic strength, and composition and concentration of the extracellularmatrix components which includes both collagen and non-collagenousmolecules. A “bioactive engineered extracellular matrix” compositionrefers to an engineered extracellular matrix composition that can bepolymerized to form a three dimensional scaffold that is capable ofremodeling tissues in vivo.

As used herein the term “naturally occurring extracellular matrix”comprises any noncellular material naturally secreted by cells (such asintestinal submucosa) isolated in their native configuration with orwithout naturally associated cells.

As used herein the term “submucosal matrices” refers to naturalextracellular matrices, known to be effective for tissue remodeling,that have been isolated in their native configuration, includingsubmucosa derived from vertebrate intestinal tissue, stomach tissue,bladder tissue, alimentary tissue, respiratory tissue and genitaltissue.

As used herein the term “exogenous” or “exogenously added” designatesthe addition of a new component to a composition, or the supplementationof an existing component already present in the composition, usingmaterial from a source external to the composition.

As used herein “sterilization” or “sterilize” or “sterilized” meansremoving unwanted contaminants including, but not limited to,endotoxins, nucleic acid contaminants, and infectious agents.

As used herein “stiffness” or elastic or linear modulus” refers to thefundamental material property defined by the slope linear portion of astress-strain curve that results from conventional mechanical testingprotocols.

As used herein, the term “purified” and like terms relate to theisolation of a molecule or compound in a form that is substantially freefrom other components with which they are naturally associated (e.g.,the total amount of nondesignated components present in the compositionrepresenting less than 5%, or more typically less than 1%, of total dryweight).

As used herein the term “three dimensional purified collagen matrix (3Dmatrix)” refers to an engineered purified collagen based matrix, asdefined above, and the fluid that surrounds the collagen fibril network.A “3D purified collagen matrix populated/seeded with cells” furthercomprises a viable population of cells contained within the matrix.

As used herein the term “three dimensional extracellular matrix (3DECM)” refers to an engineered extracellular matrix, as defined above,and the fluid that surrounds the collagen fibril network. A “3Dextracellular matrix populated/seeded with cells” further comprises aviable population of cells contained within the matrix.

As used herein the term “three dimensional matrix (3D matrix)” is ageneric term that is intended to include both “three dimensionalpurified collagen matrices (3D purified collagen matrices)” as well as“three dimensional extracellular matrices (3D ECM).

As used herein the term “collagen fibril” refers to a quasi-crystalline,filamentous structure formed by the self-assembly of soluble trimericcollagen molecules. The collagen molecules in a collagen fibriltypically pack in a quarter-staggered pattern giving the fibril acharacteristic striated appearance or banding pattern along its axis.Solubilized collagen that is assembled in vitro to form collagen fibrilsexhibit similarities to collagen structures found in vivo (Veis andGeorge, 1994 Fundamental of interstitial collagen assembly. In:Yurchenco P D, Birk D E, and Mecham R P (eds.), Extracellular MatrixAssembly and Structure, Academic Press, Inc., San Diego, pp. 15-45.).Within tissues in vivo, collagen fibrils are organized as bundles in ahierarchical manner to form fibers. Collagen fibers are furtherorganized in a tissue-specific fashion to provide tissues with specificstructural-functional properties. Collagen fibrils are distinct from theamorphous aggregates or precipitates of insoluble collagen that can beformed by dehydrating (e.g., lyophilization) collagen suspensions toform porous network scaffolds. Collagen networks formed from amorphousaggregates, or precipitates of insoluble collagen, have characteristicsthat distinct from those formed from collagen fibrils as defined above.

EMBODIMENTS

Cell culture scaffolds presenting a more biologically relevantmicroenvironment are disclosed. More particularly, these cell culturescaffolds comprise three-dimensional matrices/biomaterials that arecreated from solubilized collagen compositions. The solubilized collagencompositions are prepared from biological sources, such as naturallyoccurring extracellular matrices, including for example submucosalmatrices. More particularly, the soluble polymers suitable for use inthe present invention can be isolated, to varying degrees of purity,from natural tissues and include, but are not limited to, type Icollagen, type III collagen, growth factors and glycosaminoglycans. Inone embodiment the solubilized collagen composition comprises purifiedtype I collagen or a mixture of purified type I and type III collagen.When provided with the proper conditions, the solubilized collagencomposition undergoes polymerization/self assembly to form a threedimensional scaffold/biomaterial comprised of collagen fibrils. In oneembodiment the soluble polymers comprise type I collagen monomers, whereupon polymerization the resulting scaffolds represent a compositematerial comprising insoluble collagen fibrils and an interfibrillarfluid component, referred to herein as a three dimensional matrix.

An array of scaffolds/biomaterials can be created by varying thecomposition of ECM molecules as well as the self-assembly/polymerizationconditions. Surprisingly, applicants have discovered that upon seedingprogenitor cells or stem cells within engineered purified collagen basedmatrices (scaffolds) representing different microstructural compositions(e.g., having different dimensioned and organizations of the collagenfibrils and filaments), distinct patterns of cell survival, growth,proliferation, and differentiation are obtained. In particular,applicants have discovered that engineered purified collagen basedmatrices representing different microstructural compositions (e.g.,varied fibril dimensions (length, diameter) and densities) will impactthe rate of cell proliferation as well as the pattern of cellularcondensation, aggregation, fusion, and cellular differentiation eventsand their associated time-line. These results are significant becausethey indicate that engineered purified collagen based matrices can bespecifically designed to foster the proliferation of stem cells whilemaintaining their precursor or multi-potential status. Furthermore,engineered purified collagen based matrices can be designed to directdifferentiation of cells down a specific cell lineage (such as fat,bone, muscle, or cartilage) to form 3D organotypic tissues (that isreminiscent of in vivo tissue structure and function).

In accordance with one embodiment, stem cells and/or progenitor cellsare seeded at relatively low densities on or within the variousengineered purified collagen based matrices. It is known that, ingeneral, cell behavior is determined by a combination of signal inputsarising from soluble factors, biophysical factors, the extracellularmatrix substrate, and cell-cell interactions. Seeding cells at arelative low cell density on or within the collagen based matrices ofthe present invention allows ECM-based signaling to predominate oversignals derived from cell-cell interactions. In accordance with oneembodiment, cells are initially seeded on or within the engineeredpurified collagen based matrices at a minimal cell density that willallow for cell viability and replication (i.e., the minimalfunctionality density). This minimal functional density can be easilyestablished for the particular cell type to be cultured and for thespecific culture conditions utilized.

In accordance with one embodiment the stem cells or progenitor cells areseeded within the collagen based matrix at a cell density substantiallyhigher than the minimal functionality density but at a relative lowdensity compared to standard cell culture techniques. In one embodimentthe cells comprise stem cells, wherein the cells are seeded at a densitywithin 3 orders of magnitude of the minimal functionality density, inanother embodiment stem cells are seeded at a density within 2 orders ofmagnitude of the minimal functionality density, and in anotherembodiment the stem cells are seeded at a density within an order ofmagnitude of the minimal functionality density. The stem cells can beseeded at a relatively high density of about 1×10⁶ to about 1×10⁸cells/ml, or at a more typical density of about 1×10³ to about 1×10⁵cells/ml. Seeding the cells at the relative high density of about 1×10⁶to about 1×10⁸ cells/ml will promote cell to cell interactions over cellto matrix interactions. Accordingly, stem cells seeded at relativelyhigh densities will develop into fat tissue even when the cells arecultured within 3D matrices of high collagen fibril density. In oneembodiment stem cells are seeded at a density of less than 5×10⁴cells/ml, more typically at a density of about 5×10⁴ cells/ml. Inanother embodiment stem cells are seeded at a density of less than 1×10⁴cells/ml, in another embodiment stem cells are seeded at a densityselected from a range of about 1×10² to about 5×10³.

As disclosed herein an improved method for culturing stem cells isprovided that uses three dimensional purified collagen based matrices.The improved method allows for enhanced proliferation of stem cells aswell as better control over the differentiation of the cultured cells.In one embodiment the method comprises the steps of providing asolubilized collagen composition, adding cells to the collagencomposition, and polymerizing the solubilized collagen composition toform collagen fibrils. The solubilized collagen composition comprisescollagen that has been isolated with or without additional componentsfrom natural tissues.

In accordance with one embodiment the solubilized collagen compositionis prepared using purified type I collagen as a starting material. Inone embodiment collagen, and more particularly type I or type IIIcollagen, that has been isolated from tissues is subjected to a finalpurification step that removes any reagents that were used during theisolation steps. In one embodiment the final purification step comprisesdialyzing the isolated collagen in an aqueous solution, and in oneembodiment the isolated collagen is dialyzed against a dilute acidsolution, including for example, hydrochloric acid. In one embodimentthe final purification step comprises dialyzing the isolated collagenagainst a 0.01 N HCl solution. Isolated type I or isolated type IIIcollagen preparations are commercially available, and these commerciallyavailable materials are subjected to a further purification step,including for example, dialyzing against a dilute (about 0.001 N toabout 0.1 N) hydrochloric acid solution to produce purified collagensuitable for use for forming 3D purified collagen matrices. Thedialysate can optionally be filtered and/or centrifuged to removeparticulate matter. In accordance with one embodiment, the collagencomponent of the solubilized collagen composition consists essentiallyof purified collagen, the majority of which are in monomeric form. In afurther embodiment the composition is formed from purified collagen (themajority of which are in monomeric form) that is greater than 75% type Icollagen, or greater than 90% type I collagen. In one embodiment acomposition consisting essentially of purified collagen is dissolved inan acid solution, such as hydrochloric acid to prepare a solubilizedcollagen composition of the desired concentration. In one embodiment thepurified collagen is dissolved in about 0.001 N to about 0.1 N, fromabout 0.005 N to about 0.1 N, from about 0.005 N to about 0.01 N, fromabout 0.01 N to about 0.1 N, from about 0.05 N to about 0.1 N, fromabout 0.001 N to about 0.05 N, about 0.001 N to about 0.01 N, or fromabout 0.01 N to about 0.05 N hydrochloric acid solution.

In another embodiment, a three dimensional purified collagen matrix isprovided, wherein the matrix is formed from a solubilized collagencomposition wherein the collagen components of the solubilized collagencomposition consist essentially of purified type I and type IIIcollagen. The component fibrils of such matrices have been found to havea greater degree of flexibility relative to the fibrils of engineeredpurified collagen matrices that are formed using only type I collagen.In one embodiment the matrix comprises type I collagen and type IIIcollagen in a ratio of 200:1. The method of forming matrices withfibrils that exhibit a higher degree of flexibility comprises the stepsof combining in vitro at least 100 ug/ml of type I collagen with atleast 0.5 ug/ml of type III collagen to obtain a total amount ofcollagen, and forming in vitro a three dimensional purified collagenmatrix wherein the three dimensional matrix has decreased stiffnesscompared to a 3D matrix formed in vitro with type I collagen when thetotal amount of collagen in the two matrices is equivalent.

In another embodiment, a method of preparing an extracellular matrixcomposition is provided. The method comprises the steps of combining invitro at least 100 ug/ml of type I collagen with at least 0.5 ug/ml oftype III collagen to obtain a total amount of collagen, and forming invitro a three dimensional matrix. In one embodiment the type I and typeIII collagen is dissolved in about 0.001 N to about 0.1 N, from about0.005 N to about 0.1 N, from about 0.005 N to about 0.01 N, from about0.01 N to about 0.1 N, from about 0.05 N to about 0.1 N, from about0.001 N to about 0.05 N, about 0.001 N to about 0.01 N, or from about0.01 N to about 0.05 N hydrochloric acid solution either before or afterthe combining step.

In another embodiment, an extracellular matrix composition for use inrepairing diseased or damaged tissues is provided. The extracellularmatrix composition comprises at least 100 ug/ml of type I collagen andat least 0.5 ug/ml of type III collagen, wherein the type I collagen totype III collagen ratio is selected from the group consisting of 200:1,100:1, 50:1, 15:1, 10:1, 8:1, 6:1, 5:1, 3:1, and 2:1, and a populationof cells. The matrix is formed by provided a solubilized collagencomposition comprising type I and type III collagen, in a ratio selectedfrom the group consisting of 200:1, 100:1, 50:1, 15:1, 10:1, 8:1, 6:1,5:1, 3:1, and 2:1, polymerizing the solubilized collagen composition toform collagen fibrils, and adding cells to the collagen compositioneither before or after the polymerization step. In one embodiment acomposition comprising solubilized collagen and stem cells is injectedinto a host and the polymerization of the solubilized collagencomposition occurs in vivo to form a cell entrapping matrix.Alternatively, the solubilized collagen composition can be polymerizedin vitro and the polymerized matrix, comprising the population of cells,can be subsequently injected or implanted in a host. In anotherembodiment the population of cells entrapped within the 3D matrix can becultured in vitro, for a predetermined length of time, to increase cellnumbers and/or induce differentiation of the cell population prior toimplantation into a host. In a further embodiment, the population ofcells can be cultured in vitro, for a predetermined length of time, toincrease cell numbers and/or induce differentiation of the cellpopulation and the cells can be separated from the matrix and implantedinto the host in the absence of the polymerized matrix.

In one illustrative embodiment, the engineered purified collagen basedmatrix comprises type III collagen in the range of about 0.5% to about33% of total collagen in the matrix. In another illustrative embodiment,the engineered purified collagen based matrix comprises type I collagenin the range of about 66% to about 99.5% of total collagen in thematrix. In yet another illustrative embodiment, the type I collagen totype III collagen ratio is in the range of about 2:1 to about 200:1,wherein the type I collagen to type III collagen ratio may be selectedfrom the group consisting of 200:1, 100:1, 50:1, 15:1, 10:1, 8:1, 6:1,5:1, 3:1, and 2:1.

In another embodiment, a method of enhancing cell proliferation withinan extracellular matrix composition is provided. The method comprisesthe steps of combining in vitro an amount of type I collagen with anamount of type III collagen to obtain a total amount of collagen whereinthe ratio of type III collagen to type I collagen is at least 1:6, andforming in vitro a three-dimensional extracellular matrix wherein theextracellular matrix enhances cell proliferation compared to anextracellular matrix formed in vitro with type I collagen wherein theamount of type I collagen is equivalent to the total amount of type Icollagen in the combining step. In yet another embodiment, the methodcomprises the steps of combining in vitro at least 3 ug/ml of type Icollagen with at least 0.5 ug/ml of type III collagen to obtain a totalamount of collagen wherein the ratio of type III collagen to type Icollagen is at least 1:6, and forming in vitro a three-dimensionalextracellular matrix wherein the extracellular matrix enhances cellproliferation compared to an extracellular matrix formed in vitro withtype I collagen, wherein the amount of type I collagen is equivalent tothe total amount of type I collagen in the combining step.

In another illustrative embodiment, the method of preparing anengineered purified collagen based matrix comprises combining type I andtype III collagen wherein the type III collagen is added in the range ofabout 17% to about 33% of total collagen in the matrix. In anotherillustrative embodiment, the type I collagen is added in the range ofabout 66% to about 83% of total collagen in the matrix. In yet anotherillustrative embodiment, the type I collagen to type III collagen ratiois in the range of about 6:1 to about 1:1, wherein the type I collagento type III collagen ratio may be selected from the group consisting of6:1, 5:1, 4:1, 3:1, 2:1, and 1:1.

Applicants have also discovered that the concentration of total collagenpresent in solubilized collagen composition will impact themicrostructure of the matrix, and the behavior of stem cells culturedwithin a matrix polymerized from such a composition (see FIG. 9). 3Dmatrices can be prepared from solubilized collagen compositions havingpurified collagen concentrations ranging from as little as 0.05 mg/ml toas much as 40 mg/ml. Typically the 3D matrices are prepared frompurified solubilized collagen compositions having a collagenconcentration selected from a range of about 0.1 mg/ml to about 5.0mg/ml, and in one embodiment about 1.5 mg/ml to about 3.0 mg/ml. Table 1summarizes the effect of total collagen concentration on the fibrilstructure of the matrix:

TABLE 1 Microstructure and Mechanical Properties of 3D Purified CollagenMatrices Fibril Diameter Fibril Diameter Fibril Area Stiffness (confocal(scanning Collagen Fraction (Linear reflection electron Concentration(Density; %) Modulus; kPa) microscopy; nm) microscopy, nm) 0.3 mg/ml, pH7.4  1.54 ± 0.507  418 ± 121 1 mg/ml, pH 7.4 11.5 ± 1.9 10.7 ± 1.93 446± 65 1.5 mg/ml, pH 7.4   12 ± 1.4  8.5 ± 1.65 412.63 ± 76   115.16 ±23.18  2 mg/ml, pH 7.4  14.8 ± 4.25 16.6 ± 2.68 435 ± 61 80.8 ± 18.3 3mg/ml, pH 7.4 18.4 ± 1.9 24.3 ± 4.16 430 ± 71 2 mg/ml, pH 6  1.84 ±0.701 490 ± 96 2 mg/ml, pH 7 12.7 ± 1.18 469 ± 73 2 mg/ml, pH 7.4 16.6 ±2.68 435 ± 61 2 mg/ml, pH 8 22.5 ± 3.65 421 ± 62 2 mg/ml, pH 9 33.0 ±6.93 392 ± 65 1.5 mg/ml type I + 21.5 ± 2.6 13.3 ± 1.4  385 ± 72 87 ± 170.75 mg/ml type IIIUsing the data of Table 1 and assuming a linear relationship betweencollagen concentration and the measure properties, predictions of fibrilarea fraction and matrix stiffness can be determined as a function ofcollagen concentration using the following equations:

Fibril Area Fraction=3.6514×Collagen Concentration+7.3286

R²=0.9681

Stiffness=8.1145×Collagen Concentration−0.3306

R²=0.9304

Prediction of Stiffness as a Function of Fibril Diameter (Assumption:Fibril Area Fraction does not Change; Relationship Based Upon pH Data):

Stiffness=−0.2916×Fibril Diameter+146.02

R²=0.9581 (based upon pH data)

The 3D matrices formed in accordance with the present disclosurerepresent a matrix of collagen fibrils. The fibrils of the matrices areformed at a fibril area fraction (density) of about 7.7% to about 25%total volume. In one embodiment the 3D matrices have a fibril areafraction of about 12.8% to about 18.3% total volume. In anotherembodiment the 3D matrices have a fibril area fraction of about 18.5% toabout 25% total volume. In one embodiment the 3D matrix has a fibrilarea fraction of about 12.8% to about 18.3% total volume and the fibrilshave a hydrated diameter of about 350 to about 475 nm. In anotherembodiment the 3D matrix has a fibril area fraction of about 18.5% toabout 25% total volume and the fibrils have a hydrated diameter of about375 to about 500 nm.

Three dimensional matrices having low fibril density and low stiffnessenhance stem cell proliferation with decreased differentiation of thecells. Accordingly, 3D matrices formed from solubilized collagencompositions having about 0.1 mg/ml to about 3 mg/ml collagen, and moretypically about 0.5 mg/ml to about 2.5 mg/ml collagen are utilized tostimulate stem cell proliferation. The 3D matrices so formed will have afibril predicted fibril area fraction (density) of about 7.7% to about18.3% total volume and about 9.2% to about 16.5% total volume,respectively. In one embodiment the 3D matrices are formed fromsolubilized collagen compositions having about 3 mg/ml to about 1.5mg/ml collagen and in one embodiment the solubilized collagencompositions have about 2.5, 2.0, 1.5, or 1.0 mg/ml of collagen.Alternatively, higher concentrations of total collagen present in thethree dimensional matrix leads to differentiation of stem cells.Accordingly, 3D matrices (having a fibril area fraction of at leastabout 18% total volume) formed from solubilized collagen compositionshaving more than about 3 mg/ml are utilized to stimulate differentiationof stem cells cultured within the matrix. In one embodiment the 3Dmatrices are formed from solubilized collagen compositions having about3.2, 3.4, 3.6, 3.8, 4.0, 4.5 or 5.0 mg/ml of collagen, resulting in 3Dmatrices having a fibril area fraction of about 19%, 19.7%, 20.5%,21.2%, 22%, 23.8% and 25.6% total volume, respectively.

As reported herein the relative stiffness (elastic or linear modulus) ofa 3D matrix can be modified by controlling the relative proportion oftype I to type III collagen, the fibril area fraction (density), or thefibril diameter of the collagen fibrils in the 3D matrix. In accordancewith one embodiment 3D matrices are prepared having a relatively lowstiffness (elastic or linear modulus) of about 0.48 to about 24.0 kPa.In one embodiment these matrices are used to propagate stem cells andprogenitor cells without further differentiation of the cells and/ortheir progeny. In another embodiment 3D matrices are prepared having arelatively high stiffness of about 25 to about 40 kPa. In one embodimentthese relatively stiffer matrices are used to induce the differentiationof stem cells and progenitor cells and/or their progeny. In oneembodiment a 3D matrix is provided having a relatively low stiffness ofabout 0.48 to about 24.0 kPa and a relatively low fibril area fraction(density) of about 7% to about 18% total volume. In an alternativeembodiment a 3D matrix is provided having a relatively high stiffness ofabout 25 to about 40 kPa and a relatively high fibril area fraction(density) of about 19% to about 26% total volume.

In another embodiment the solubilized collagen composition comprisescollagen monomers isolated from natural tissues, and includes additionalcomponents that are naturally associated with the native tissues and/orexogenously added components. In one embodiment various exogenousmaterials, such as growth factors are added to the collagen basedmatrices of the present invention. In one embodiment the solubilizedcollagen composition represents a solubilized fraction of a naturallyoccurring extracellular matrix, and in one embodiment the naturallyoccurring extracellular matrix is a vertebrate submucosal matrix. In oneembodiment the solubilized collagen composition represents a solubilizedfraction of vertebrate intestinal submucosa.

In other embodiments, acetic acid, formic acid, lactic acid, citricacid, sulfuric acid, ethanoic acid, carbonic acid, nitric acid, orphosphoric acid can be used to solubilize the naturally occurringextracellular matrix (or a purified lyophilized collagen composition) toproduce a solubilized collagen composition. The solubilized collagencomposition derived from a naturally occurring extracellular matrix,such as vertebrate intestinal submucosa, can then be polymerized to forman engineered extracellular matrix.

The invention also relates to methods of preparation and compositionscomprising solubilized extracellular matrix components polymerized invitro where the extracellular matrix components are solubilized by othermethods known in the art. The polymerizing step can be performed underconditions that are systematically varied where the conditions areselected from the group consisting of pH, phosphate concentration,temperature, buffer composition, ionic strength, the extracellularmatrix components in the solubilized extracellular matrix composition,and the concentration of the extracellular matrix components in thesolubilized extracellular matrix composition.

In accordance with one embodiment a method of forming a 3D matrixcomprising stem cells is provided. The method comprises the steps ofproviding an acid solubilized purified type I collagen composition. Inone embodiment the collagen composition further comprises type IIIcollagen. In one embodiment the purified collagen represents acommercially available isolated preparation of collagen that is furthersubjected to purification, including for example dialyzing against ansolution of about 0.005 N to about 0.1 N HCl, more typically about 0.01N HCl. Typically the solubilized collagen composition comprises purifiedcollagen that is suspended in about 0.005 N to about 0.1 N HCl solution,and in one embodiment suspended in 0.01N HCl. The solubilized collagencomposition is also typically sterilized using standard techniquesincluding for example contact with chloroform or peracetic acid. Stemcells are then added to the solubilized collagen composition at aspecific density, typically ranging from about 1×10³ to about 1×10⁸. Inone embodiment the stem cells are added to the solubilized collagencomposition at a concentration of less than 5×10⁴ cells per milliliter,and in one embodiment the cells are added at a density of about 10 toabout 10³ per milliliter. In accordance with one embodiment thecollagen/cell suspension is then pipetted into a well plate and allowedto polymerize in a humidified environment at 37° C. for approximately 30minutes. In an alternative embodiment the collagen/cell suspension isinjected into a host and the composition is polymerized in vivo.

As noted above solubilized collagen compositions can be prepared fromvertebrate submucosal matrices wherein the collagen compositionscomprise additional components besides collagen. Vertebrate submucosalmatrices can be obtained from various sources, including intestinaltissue harvested from animals raised for meat production, including, forexample, pigs, cattle and sheep or other warm-blooded vertebrates.According to one embodiment the solubilized collagen composition isderived from one or more sources selected from the group consisting ofintestinal submucosa, stomach submucosa, urinary bladder submucosa,uterine submucosa, and any other submucosal material that can be used toremodel endogenous tissue.

In one embodiment the submucosa comprises the tunica submucosadelaminated from both the tunica muscularis and at least the luminalportion of the tunica mucosa of a warm-blooded vertebrate. Suchconstructs can be prepared by mechanically removing the luminal portionof the mucosa and the external muscle layers and lysing resident cellswith hypotonic washes.

It is known that compositions comprising the tunica submucosadelaminated from both the tunica muscularis and at least the luminalportion of the tunica mucosa of the submucosal tissue of warm-bloodedvertebrates can be used as tissue graft materials (see, for example,U.S. Pat. Nos. 4,902,508 and 5,281,422 incorporated herein byreference). Such submucosal tissue preparations are characterized byexcellent mechanical properties, including high compliance, high tensilestrength, a high burst pressure point, and tear-resistance.

Submucosa-derived matrices are collagen based biodegradable matricescomprising highly conserved collagens, glycoproteins, proteoglycans, andglycosaminoglycans in their natural configuration and naturalconcentration. Such submucosal material serves as a matrix for theregrowth of endogenous tissues at the implantation site (e.g.,biological remodeling). The submucosal material serves as a rapidlyvascularized matrix for support and growth of new endogenous connectivetissue. Thus, submucosa matrices have been found to be trophic for hostcells of tissues to which it is attached or otherwise associated in itsimplanted environment. In multiple experiments submucosal tissue hasbeen found to be remodeled (resorbed and replaced with autogenousdifferentiated tissue) to assume the characterizing features of thetissue(s) with which it is associated at the site of implantation orinsertion.

Small intestinal submucosa tissue is an illustrative source ofsubmucosal tissue for use in this invention. Submucosal tissue can beobtained from various sources, for example, intestinal tissue can beharvested from animals raised for meat production, including, pigs,cattle and sheep or other warm-blooded vertebrates. Small intestinalsubmucosal tissue is a plentiful by-product of commercial meatproduction operations and is, thus, a low cost material.

Suitable intestinal submucosal tissue typically comprises the tunicasubmucosa delaminated from both the tunica muscularis and at least theluminal portion of the tunica mucosa, but other tissue constructs canalso be used. In one illustrative embodiment the intestinal submucosaltissue comprises the tunica submucosa and basilar portions of the tunicamucosa including the lamina muscularis mucosa and the stratum compactumwhich layers are known to vary in thickness and in definition dependenton the source vertebrate species.

The preparation of submucosal tissue is described in U.S. Pat. No.4,902,508, the disclosure of which is expressly incorporated herein byreference. A segment of vertebrate intestine, for example, preferablyharvested from porcine, ovine or bovine species, but not excluding otherspecies, is subjected to abrasion using a longitudinal wiping motion toremove the outer layers, comprising smooth muscle tissues, and theinnermost layer, i.e., the luminal portion of the tunica mucosa. Thesubmucosal tissue is rinsed under hypotonic conditions, such as withwater or with saline under hypotonic conditions, and is optionallysterilized.

The submucosal tissue can be sterilized using conventional sterilizationtechniques including glutaraldehyde tanning, formaldehyde tanning atacidic pH, propylene oxide or ethylene oxide treatment, gas plasmasterilization, gamma radiation, electron beam, and/or peracetic acidsterilization. Sterilization techniques which do not adversely affectthe structure and biotropic properties of the submucosal tissue can beused. An illustrative sterilization technique is exposing the submucosaltissue to peracetic acid, 1-4 Mrads gamma irradiation (or 1-2.5 Mrads ofgamma irradiation), ethylene oxide treatment, exposure to chloroform, orgas plasma sterilization. The submucosal tissue can be subjected to oneor more sterilization processes. In illustrative embodiments, the intactextracellular matrix material can be sterilized with peracetic acid orthe solubilized collagen composition can be sterilized. The submucosaltissue can be subjected to one or more sterilization processes. Thesubmucosal tissue can be stored in a hydrated or dehydrated state priorto solubilization in accordance with the invention.

Extracellular matrix-derived tissues other than intestinal submucosatissue may be used in accordance with the methods described herein andused as a source for preparing solubilized collagen compositions.Methods of preparing and treating other extracellular matrix-derivedtissues are known to those skilled in the art and may be similar to themethods described above. For example, see U.S. Pat. Nos. 5,163,955(pericardial tissue), 5,554,389 (urinary bladder submucosa tissue),6,099,567 (stomach submucosa tissues), 6,576,265 (extracellular matrixtissues generally), 6,793,939 (liver basement membrane tissues), andU.S. patent application publication no. US 2005/0019419 A1 (liverbasement membrane tissues), and WO 01/45765 (extracellular matrixtissues generally), each incorporated herein by reference. Thepreparation and use of submucosa tissues as graft compositions is alsodescribed in U.S. Pat. Nos. 4,902,508, 5,281,422, and 5,275,826, eachincorporated herein by reference.

In one illustrative embodiment, the extracellular matrix material issolubilized with an acid and the solubilized fraction is recovered forpolymerization to form the collagen based matrices of the presentinvention. Typically, prior to solubilization, the source extracellularmatrix material is comminuted by tearing, cutting, grinding, or shearingthe harvested extracellular matrix material. In one illustrativeembodiment, the extracellular matrix material can be comminuted byshearing in a high-speed blender, or by grinding the extracellularmatrix material in a frozen or freeze-dried state, and then lyophilizingthe material to produce a powder having particles ranging in size fromabout 0.1 mm² to about 1.0 mm². The extracellular matrix material powdercan thereafter be hydrated with, for example, water or buffered salineto form a fluid or liquid or paste-like consistency. In one illustrativeembodiment, the extracellular matrix tissue is comminuted by freezingand pulverizing under liquid nitrogen in an industrial blender. Thepreparation of fluidized forms of the source extracellular matrixmaterial, such as submucosa tissue, is described in U.S. Pat. No.5,275,826, the disclosure of which is expressly incorporated herein byreference.

In one illustrative embodiment, an acid, such as hydrochloric acid,acetic acid, formic acid, sulfuric acid, ethanoic acid, carbonic acid,nitric acid, or phosphoric acid, is used to solubilize the sourceextracellular matrix material. In various illustrative embodiments, theacidic conditions for solubilization can include solubilization at about0° C. to about 60° C., and incubation periods of about 5 minutes toabout 96 hours. In other illustrative embodiments, the concentration ofthe acid, such as hydrochloric acid, can be from about 0.001 N to about0.1 N, from about 0.005 N to about 0.1 N, from about 0.01 N to about 0.1N, from about 0.05 N to about 0.1 N, from about 0.001 N to about 0.05 N,about 0.001 N to about 0.01 N, or from about 0.01 N to about 0.05 M.However, the solubilization can be conducted at any temperature, for anylength of time, and at any concentration of acid.

Any of the source extracellular matrix materials described above can beused and the solubilization step can be performed in the presence of anacid or in the presence of an acid and an enzyme. The acidsolubilization step results in a solubilized extracellular matrixcomposition that remains bioactive (i.e., is capable of polymerizing andremodeling tissues in vivo) after lyophilization.

In one illustrative embodiment, the extracellular matrix material istreated with one or more enzymes before, during, or after the acidsolubilization step. For enzymes that are inactive at acidic pH, forexample, the extracellular matrix material is treated with the enzymebefore the acid solubilization step or after the acid solubilizationstep, but under conditions that are not acidic. Enzymatic digestion ofthe extracellular matrix material is conducted under conditions that areoptimal for the specific enzyme used and under conditions that retainthe ability of the solubilized components of the extracellular matrixmaterial to polymerize. The concentration of the enzyme depends on thespecific enzyme used, the amount of extracellular matrix material to bedigested, the desired time of digestion, and the desired temperature ofthe reaction. In various illustrative embodiments, about 0.01% to about0.5% (weight per volume, such that 0.01% is equivalent to 0.01 g/100 ml)of enzyme is used. Exemplary enzymes include pepsin, bromelain,cathepsins, chymotrypsin, elastase, papain, plasmin, subtilisin,thrombin, trypsin, matrix metalloproteinases (e.g., stromelysin,elastase), glycosaminoglycan-specific enzymes (e.g., chondroitinase,hyaluronidase, heparinase) and the like, or combinations thereof. Thesource extracellular matrix material can be treated with one or moreenzymes. In illustrative embodiments, the enzyme digestion can beperformed at about 2° C. to about 37° C. However, the digestion can beconducted at any temperature, for any length of time (e.g., about 5minutes to about 96 hours), and at any enzyme concentration.

In illustrative embodiments, the ratio of the extracellular matrixmaterial (hydrated) to total enzyme (weight/weight) ranges from about 25to about 2500. If an enzyme is used, it should be removed (e.g., byfractionation) or inactivated after the desired incubation period forthe digestion so as to not compromise stability of the components in thesolubilized extracellular matrix composition. Enzymes, such as pepsinfor example, can be inactivated with protease inhibitors, a shift toneutral pH, a drop in temperature below 0° C., or heat inactivation, ora combination of these methods.

In another illustrative embodiment, the source extracellular matrixmaterial can be extracted in addition to being solubilized withhydrochloric acid. Extraction methods for extracellular matrices areknown to those skilled in the art and are described in detail in U.S.Pat. No. 6,375,989, incorporated herein by reference. Illustrativeextraction excipients include, for example, chaotropic agents such asurea, guanidine, sodium chloride, magnesium chloride, and non-ionic orionic surfactants.

In one embodiment, the solubilized collagen composition comprisessoluble and insoluble components, and at least a portion of theinsoluble components of the solubilized collagen composition can beseparated from the soluble components. For example, the insolublecomponents can be separated from the soluble components bycentrifugation (e.g., at 12,000 rpm for 20 minutes at 4° C.). Inalternative embodiments, other separation techniques known to thoseskilled in the art, such as filtration, can be used.

In accordance with one illustrative embodiment, the solubilizedextracellular matrix composition, prepared with or without theabove-described separation step, is fractionated prior topolymerization. In one illustrative aspect, the solubilizedextracellular matrix composition can be fractionated by dialysis.Exemplary molecular weight cut-offs for the dialysis tubing or membraneare from about 3,500 to about 12,000 or about 3,500 to about 5,000. Inone embodiment, the solubilized extracellular matrix composition isdialyzed against an acidic solution having a low ionic strength. Forexample, the solubilized extracellular matrix composition can bedialyzed against a hydrochloric acid solution, however any other acidscan be used, including acetic acid, formic acid, citric acid, lacticacid, sulfuric acid, ethanoic acid, carbonic acid, nitric acid, orphosphoric acid. In another example, the extracellular matrixcomposition can be dialyzed against water as long as the pH isapproximately 6 or below.

In various illustrative embodiments, the fractionation, for example bydialysis, can be performed at about 2° C. to about 37° C. for about 1hour to about 96 hours. In another illustrative embodiment, theconcentration of the acid, such as acetic acid, hydrochloric acid,formic acid, citric acid, lactic acid, sulfuric acid, ethanoic acid,carbonic acid, nitric acid, or phosphoric acid, against which thesolubilized extracellular matrix composition is dialyzed, can be fromabout 0.001 N to about 0.1 N, from about 0.005 N to about 0.1 N, fromabout 0.01 N to about 0.1 N, from about 0.05 N to about 0.1 N, fromabout 0.001 N to about 0.05 N, about 0.001 N to about 0.01 N, or fromabout 0.01 N to about 0.05 N. In one illustrative embodiment, thesolubilized extracellular matrix composition is dialyzed against 0.01 NHCl. However, the fractionation can be performed at any temperature, forany length of time, and against any concentration of acid.

In accordance with one embodiment the 3D matrix used for culturing stemcells comprises a lyophilized, solubilized collagen composition that isrehydrated prior to contact with the cells. As discussed above, the term“lyophilized” means that water is removed from the composition,typically by freeze-drying under a vacuum (typically to dryness). In oneillustrative aspect, a solubilized extracellular matrix composition islyophilized after solubilization. In another illustrative aspect, thesolubilized extracellular matrix composition is lyophilized after thesolubilized portions have been separated from the insoluble portions. Inyet another illustrative aspect, the solubilized extracellular matrixcomposition is lyophilized after a fractionation step but prior topolymerization. In another illustrative embodiment, the polymerizedmatrix is lyophilized. In one illustrative lyophilization embodiment,the solubilized extracellular matrix composition is first frozen, andthen placed under a vacuum. In another lyophilization embodiment, thesolubilized extracellular matrix composition is freeze-dried under avacuum. Any method of lyophilization known to the skilled artisan can beused.

In accordance with one embodiment, the solubilized collagen compositionis sterilized before polymerization. In one embodiment the source of thesolubilized collagen (e.g., a naturally occurring extracellular matrix,or a lyophilized purified collagen composition) is sterilized prior tothe solubilization step. Sterilization of the extracellular matrixmaterial can be performed, for example, as described in U.S. Pat. Nos.4,902,508 and 6,206,931, incorporated herein by reference. In anotherembodiment, the solubilized collagen composition is directly sterilized,for example, with peracetic acid. In one embodiment wherein anextracellular matrix is solubilized with an acid and the resultingmaterial is fractionated to isolate a fraction comprising solubilizedcollagen, sterilization can be carried out either before or after thefractionation step. In another illustrative embodiment, the lyophilizedcomposition itself is sterilized before rehydration, for example usingan e-beam sterilization technique. In yet another illustrativeembodiment, the polymerized matrix formed from the components of thesolubilized collagen matrix composition is sterilized.

In one illustrative embodiment, the solubilized extracellular matrixcomposition is directly sterilized before the fractionation/separationstep, for example, with peracetic acid or with peracetic acid andethanol (e.g., by the addition of 0.18% peracetic acid and 4.8% ethanolto the solubilized extracellular matrix composition before theseparation step). In another embodiment, sterilization can be carriedout during the fractionation step. For example, the solubilizedextracellular matrix composition can be dialyzed against chloroform,peracetic acid, or a solution of peracetic acid and ethanol to disinfector sterilize the solubilized extracellular matrix composition. Forexample, the solubilized extracellular matrix composition can besterilized by dialysis against a solution of peracetic acid and ethanol(e.g., 0.18% peracetic acid and 4.8% ethanol). The chloroform, peraceticacid, or peracetic acid/ethanol can be removed prior to polymerizationof the solubilized collagen composition, for example by dialysis againstan acid, such as 0.01 N HCl.

If the solubilized collagen composition is lyophilized, the lyophilizedcollagen matrix composition can be stored frozen or at room temperature(for example, at about −80° C. to about 25° C.). Storage temperaturesare selected to stabilize the components of the solubilized collagenmatrix composition. The compositions can be stored for about 1-26 weeks,or longer. In one illustrative embodiment, the storage solvent ishydrochloric acid. As described herein, “storage solvent” means thesolvent that the solubilized collagen matrix composition is in prior toand during lyophilization. For example, hydrochloric acid, or otheracids, at concentrations of from about 0.001 N to about 0.1 N, fromabout 0.005 N to about 0.1 N, from about 0.01 N to about 0.1 N, fromabout 0.05 N to about 0.1 N, from about 0.001 N to about 0.05 N, fromabout 0.001 N to about 0.01 N, or from about 0.01 N to about 0.05 N canbe used as the storage solvent for the lyophilized, solubilized collagenmatrix composition. Other acids can be used as the storage solventincluding acetic acid, formic acid, citric acid, lactic acid, sulfuricacid, ethanoic acid, carbonic acid, nitric acid, or phosphoric acid, andthese acids can be used at any of the above-described concentrations. Inone illustrative embodiment, the lyophilizate can be stored (i.e.,lyophilized in) an acid, such as acetic acid, at a concentration of fromabout 0.001 M to about 0.5 M or from about 0.01 M to about 0.5 M. Inanother embodiment, the lyophilizate can be stored in water with a pH ofabout 6 or below. In other illustrative embodiments, lyoprotectants,cryoprotectants, lyophilization accelerators, or crystallizingexcipients (e.g., ethanol, isopropanol, mannitol, trehalose, maltose,sucrose, tert-butanol, and Tween 20), or combinations thereof, and thelike can be present during lyophilization.

In one embodiment, the sterilized, solubilized collagen composition canbe dialyzed against 0.01 N HCl, for example, prior to lyophilization toremove the sterilization solution and so that the solubilizedextracellular matrix composition is in a 0.01 N HCl solution as astorage solvent. Alternatively, the solubilized extracellular matrixcomposition can be dialyzed against acetic acid as the storage solvent,for example, prior to lyophilization and can be lyophilized in aceticacid and redissolved in HCl or water.

If the solubilized extracellular matrix composition is lyophilized, theresulting lyophilizate can be redissolved in any solution, but may beredissolved in an acidic solution or water. The lyophilizate can beredissolved in, for example, acetic acid, hydrochloric acid, formicacid, citric acid, lactic acid, sulfuric acid, ethanoic acid, carbonicacid, nitric acid, or phosphoric acid, at any of the above-describedconcentrations, or can be redissolved in water. In one illustrativeembodiment the lyophilizate is redissolved in 0.01 N HCl. For use inproducing engineered matrices that can be injected in vivo or used forother purposes in vitro, the redissolved lyophilizate can be subjectedto varying conditions (e.g., pH, phosphate concentration, temperature,buffer composition, ionic strength, and composition and concentration ofsolubilized extracellular matrix composition components (dry weight/ml))that result in polymerization to form engineered extracellular matricesfor specific tissue graft applications.

Accordingly, in one illustrative embodiment of the method describedherein, a solubilized collagen composition is prepared by enzymaticallytreating the source extracellular matrix material with 0.1% (w/v) pepsinin 0.01 N HCl to initially solubilized the extracellular matrixmaterial, centrifuging the enzymatically treated composition at 12,000rpm for 20 minutes at 4° C. to remove insoluble components,fractionating the soluble fraction by dialysis against a 0.01 N HClsolution, and then polymerizing the dialyzed fraction.

In another illustrative embodiment, the method does not involve afractionation step. In this embodiment, the source extracellular matrixmaterial is enzymatically treated with 0.1% (w/v) pepsin in a 0.01 Nhydrochloric acid solution to produce a solubilized collagencomposition, the solubilized composition is then centrifuged to removeinsoluble components, and then the solubilized fraction is polymerized.

In another illustrative embodiment, a solubilized collagen compositionis prepared by grinding source vertebrate submucosa into a powder andenzymatically digesting the powderized submucosa with 0.1% w/v pepsinand solubilizing in 0.01 N HCl for one to three days at 4° C. Followingdigestion and solubilization, the solubilized components of thesolubilized submucosa composition are separated from the insolublecomponents by centrifugation at 12,000 rpm at 4° C. for 20 minutes. Thesupernatant, comprising the soluble components, is recovered and thepellet containing insoluble components is discarded. The supernatant isthen fractionated by dialyzing the solubilized submucosa compositionagainst 0.01 N HCl. In one embodiment, the solubilized submucosacomposition is dialyzed against several changes of 0.01 N hydrochloricacid at 4° C. using dialysis membranes having a molecular weight cut-offof 3500. Thus, the solubilized submucosa composition is fractionated toremove components having a molecular weight of less than about 3500.Alternatively, dialysis tubing or membranes having a different molecularweight cut-off can be used. The fractionated solubilized submucosacomposition is then polymerized to produce the collagen based matricesof the present invention.

In accordance with another illustrative embodiment, a solubilizedcollagen composition is prepared by grinding vertebrate submucosa into apowder and digesting the powderized submucosa composition with 0.1% w/vpepsin and solubilizing in 0.01 N hydrochloric acid for one to threedays at 4° C. The solubilized components are then separated from theinsoluble components, for example, by centrifugation at 12,000 rpm at 4°C. for 20 minutes. The supernatant, comprising the soluble components,is recovered and the pellet containing insoluble components isdiscarded. The non-fractionated solubilized submucosa composition isthen polymerized.

The present invention encompasses the formation of a solubilizedcollagen composition from a complex extracellular matrix materialwithout purification of the matrix components. However, the componentsof the naturally occurring extracellular matrices can be partiallypurified and the partially purified composition can be used inaccordance with the methods described herein to prepare a solubilizedcollagen composition. Purification methods for extracellular matrixcomponents are known to those skilled in the art and are described indetail in U.S. Pat. No. 6,375,989, incorporated herein by reference. Inaccordance with one embodiment the solubilized collagen compositionincludes purified type I collagen or type I and type III collagen as theonly protein constituents of the composition.

The solubilized collagen composition can be polymerized under differentconditions to produce a collagen based matrix having the desiredmicrostrutures and mechanical properties. Polymerization of purifiedtype I collagen solutions at different concentrations of collagenaffected fibril density while maintaining a relatively constant fibrildiameter. In addition, both fibril length and diameter are affected byaltering the pH of the polymerization reaction.

Additional conditions can be varied during the polymerization reactionto provide engineered purified collagen matrices that have the desiredproperties. In illustrative embodiments, the conditions that can bevaried include pH, phosphate concentration, temperature, buffercomposition, ionic strength, the extracellular matrix components in thesolubilized extracellular matrix composition, and the concentration ofsolubilized extracellular matrix composition components (dry weight/ml).These conditions result in polymerization of the extracellular matrixcomponents to form engineered extracellular matrices with desiredcompositional, microstructural, and mechanical characteristics.Illustratively, these compositional, microstructural, and mechanicalcharacteristics can include fibril length, fibril diameter, number offibril-fibril connections, fibril density, fibril organization, matrixcomposition, 3-dimensional shape or form, viscoelastic, tensile, orcompressive behavior, shear (e.g., failure stress, failure strain, andmodulus), permeability, swelling, hydration properties (e.g., rate andswelling), and in vivo tissue remodeling and bulking properties, anddesired in vitro cell responses. The matrices described herein havedesirable biocompatibility, vascularization, remodeling, and bulkingproperties, among other desirable properties.

In various illustrative embodiments, qualitative and quantitativemicrostructural characteristics of the engineered matrices can bedetermined by environmental or cryostage scanning electron microscopy,transmission electron microscopy, confocal microscopy, second harmonicgeneration multi-photon microscopy. In another embodiment,polymerization kinetics may be determined by spectrophotometry ortime-lapse confocal reflection microscopy. In another embodiment,tensile, compressive and viscoelastic properties can be determined byrheometry or uniaxial tensile testing. In another embodiment, a ratsubcutaneous injection model can be used to determine remodeling andbulking properties. All of these methods are known in the art or arefurther described in Examples 5-7 or are described in Roeder et al., J.Biomech. Eng. vol. 124, pp. 214-222 (2002) and in Pizzo et al., J. Appl.Physiol., vol. 98, pp. 1-13 (2004), incorporated herein by reference.

In accordance with one embodiment, the solubilized collagen compositionis polymerized at a final total collagen concentration of about 1 toabout 40 mg/ml, and in one embodiment about 1 to about 30 mg/ml, inanother embodiment about 2 to about 25 mg/ml and in another embodimentabout 5 to about 15 mg/ml. In one embodiment the final total collagen isselected from a range of about 0.25 to about 5.0 mg/ml, or in anotherembodiment the final total collagen concentration is selected from therange of about 0.5 to about 4.0 mg/ml, and in another embodiment thefinal total collagen concentration is selected from the range of about1.0 to about 3.0 mg/ml, and in another embodiment the final totalcollagen concentration is about 0.3, 0.5, 1.0, 2.0 or 3.0 mg/ml. Inother embodiments, the components of the solubilized extracellularmatrix composition are polymerized at final concentrations (dryweight/ml) of about 0.25 to about 10 mg/ml, about 0.25 to about 20mg/ml, about 0.25 to about 30 mg/ml, about 0.25 to about 40 mg/ml, about0.25 to about 50 mg/ml, about 0.25 to about 60 mg/ml, or about 0.25 toabout 80 mg/ml.

In various illustrative embodiments, the total collagen comprising thesolubilized collagen composition comprises type I and type III collagen,wherein the percent range of the type III collagen and type I collagenis selected from about 17-33% and about 66-83%, respectively, to achievevarious collagen type I/III ratios. Examples of percentage ranges oftype III collagen and type I collagen, respectively that may be used inthe matrices include 17% and 83%; 20% and 80%; 25% and 75%; 30% and 70%;and 33% and 66%, respectively. In various illustrative embodiments, thetype I collagen to type III collagen ratio may be in the range of about6:1 to about 1:1. Examples of the type I collagen to type III collagenratios that may be used in the matrices include 6:1, 5:1, 4:1, 3:1, 2:1,1.5:1, and 1:1.

In various illustrative embodiments, at least 3 ug/ml of type I collagenis combined with at least 0.5 ug/ml of type III collagen to obtain atotal amount of collagen. Examples of the amount of type I collagencombined with type III collagen, respectively, that may be used in thematrices include 3 ug/ml and 0.5 ug/ml; 1500 ug/ml and 250 ug/ml; 1500ug/ml and 500 ug/ml; 1500 ug/ml and 750 ug/ml; and 1500 ug/ml and 1500ug/ml.

In various illustrative embodiments, the conditions for combining type Icollagen and type III collagen can be the same as those described abovefor the method of decreasing stiffness of an extracellular matrixcomposition.

Illustratively, the matrix compositions produced by the methodsdescribed herein can be combined, prior to, during, or afterpolymerization, with stem cells or progenitor cells, to further enhancethe repair or replacement of diseased or damaged tissues. Examples ofprogenitor cells include those that give rise to blood cells,fibroblasts, endothelial cells, epithelial cells, smooth muscle cells,skeletal muscle cells, cardiac muscle cells, multi-potential progenitorcells, pericytes, and osteogenic cells. The population of progenitorcells can be selected based on the cell type of the intended tissue tobe repaired. For example, if skin is to be repaired, the population ofprogenitor cells will give rise to non-keratinized epithelial cells orif cardiac tissue is to be repaired, the progenitor cells can producecardiac muscle cells. The matrix composition can also be seeded withautogenous cells isolated from the patient to be treated. In analternative embodiment the cells may be xenogeneic or allogeneic innature.

In any of the embodiments described above using purified collagen, thepurified collagen can be sterilized after purification. In yet otherembodiments, the collagen that is purified can be sterilized before orduring the purification process. In other embodiments, purified collagencan be sterilized before polymerization or the matrix can be sterilizedafter polymerization.

It has been reported that the use of progenitor or stem cells to treatdamaged tissues (including for example treating myocardial infarctionfollowed by heart failure) has demonstrated early evidence of potentialutility. However, recent data, has revealed three key issues thatsignificantly limit successful delivery of reparative cells to tissues.These are 1.) inefficient and inconsistent local retention of cellsacutely following injection into tissues [Hou et al., 2005, Circulation,112:1150-6]; 2.) limited survival of cells over time following injectioninto tissues [Rehman et al., 2004, Circulation 109: 1292-8]; and 3.)lack of a suitable cellular microenvironment to modulate differentiationinto the desired tissue types (e.g., either vascular structures ormyocytes in the context of tissue remodeling in response to ischemicinsult) [Reinlib and Field, 2000, Circulation 101: E182-E187].

In accordance with one embodiment a novel cell delivery strategy isprovided that involves the suspension of cells in a liquid-phase,injectable solubilized collagen composition that polymerizes in situ toform a three-dimensional (3D) matrix. The 3D matrix is designed to bothentrap cells and provide them with an “instructive” microenvironmentwhich promotes cell survival and modulates their fate. It is anticipatedthat the introduction of cells in the presence of a comparativelyviscous medium (i.e., the solubilized collagen composition, which willsubsequently assemble in situ shortly after post-injection) will enhancethe cells local retention. Furthermore, as noted in Examples 12-15, thecomponents of the 3D matrix and their microstructural organization playan important role in determining cell fate with respect to survival,proliferation, and differentiation. Interestingly, recent data showsthat a nanofiber microenvironment formed intramyocardially followinginjection of a peptide (8-16 amino acids long) hydrogel (of which thebiological signaling capacity and degradation properties have yet to beelucidated) resulted in formation of a nanofiber microenvironment thatpromoted endogenous cell recruitment [Davis et al., 2005 Circulation111:442-50]. Furthermore, co-culture of endothelial cells withcardiomyocytes within the peptide hydrogel in vitro dramaticallydecreased apoptosis and necrosis of cardiomyocytes [Narmaneva et al.,2004 Circulation 110:962-968].

As reported herein, the biophysical signals provided by a 3Dself-assembled collagen microenvironment can be used to direct theproliferation and differentiation capacity of multi-potential, bonemarrow-derived stem cells. For example, 3D purified collagen matricescharacterized by a relatively high fibril density and stiffnesssupported an increase in clonal growth and enhanced osteogenesis (boneformation). Collectively, these results demonstrate the ability toengineer injectable, self-assembling 3D purified collagen matrices inwhich the composition, microstructure, and mechanical properties aredefined and systematically varied with discrete outcomes. In general,the biophysical features of the 3D matrix, in addition to cellularsignaling modalities consisting of soluble factors and cell-cellinteractions, are determinants of cell fate and represent a new targetfor therapeutic manipulation.

In accordance with one embodiment a method of enhancing the repair ofdamaged, diseased or congenital defective tissues is provided. Themethod comprises the steps of suspending a population of cells within asolubilized collagen composition, inducing the polymerization of thesolubilized collagen composition, and injecting the composition intowarm blooded species. Typically, the composition is injected into amammalian species, including a human for example, and in one embodimentthe cells represent autologous. In an alternative embodiment the cellsmay be xenogeneic or allogeneic cells. The injected solubilized collagencomposition polymerizes in vivo to form a 3D matrix with the populationof cells embedded within the collagen matrix. In one embodiment thepopulation of cells comprise stem cells. In one embodiment the solublecollagen composition comprises purified type I collagen, glucose, andcalcium chloride. In one embodiment a 3D purified collagen matrix isprovided comprising collagen fibrils at a fibril area fraction of about12% to about 25% (area of fibril to total area) that comprises glucoseand CaCl₂. In one embodiment the solubilized collagen compositioncomprises about 0.05 mg/ml to about 5 mg/ml total purified collagen(either type I alone or a combination of type I and type III collagen)about 1.11 mM to about 277.5 mM glucose and about 0.2 mM to about 4.0 mMCaCl₂. Applicants have discovered that the inclusion of glucose andCaCl₂ within the interstitial fluid of the 3D matrices enhances thesurvival and functioning of cells seeded within the 3D matrix.

In one embodiment the solubilized collagen composition comprises about0.1 mg/ml to about 3 mg/ml total purified collagen (either type I aloneor a combination of type I and type III collagen) in about 0.05 to about0.005N HCl (and in one embodiment about 0.01N HCl), about 0.07M to about0.28M NaCl (and in one embodiment about 0.137M NaCl), about 1.3 to about4.5 mM KCl (and in one embodiment about 2.7 mM KCl), about 4.0 to about16 mM Na₂HPO₄ (and in one embodiment about 8.1 mM Na₂HPO₄), about 0.7 toabout 3.0 mM KH₂PO₄ (and in one embodiment about 1.5 mM KH₂PO₄), about0.25 to about 11.0 mM MgCl₂ (and in one embodiment about 0.5 mM MgCl₂),about 2.8 mM to about 166 mM glucose, (and in one embodiment about 5 mMglucose). Polymerization of the solubilized collagen composition isinduced by the addition of a neutralizing solution such as NaOH. Forexample a NaOH solution can be added to a final concentration of 0.01NNaOH. The cells are then added to the composition after the addition ofneutralizing solution. In accordance with one embodiment a calciumchloride solution is also added to the solubilized collagen composition.In this embodiment, calcium chloride is added to bring the finalconcentration of CaCl₂ in the solubilized collagen composition to about0.4 mM to about 2.0 mM CaCl₂ (and in one embodiment about 0.9 mM CaCl₂).The composition is then allowed to polymerize either in vitro or in vivoto form a 3D matrix comprised of collagen fibrils wherein the cells areembedded within the 3D matrix. In illustrative embodiments thepolymerization reaction is conducted in a buffered solution using anybiologically compatible buffer system known to those skilled in the art.For example the buffer may be selected from the group consisting ofphosphate buffer saline (PBS), Tris (hydroxymethyl)aminomethaneHydrochloride (Tris-HCl), 3-(N-Morpholino) Propanesulfonic Acid (MOPS),piperazine-n,n′-bis(2-ethanesulfonic acid) (PIPES),[n-(2-Acetamido)]-2-Aminoethanesulfonic Acid (ACES),N-[2-hydroxyethyl]piperazine-N′-[2-ethanesulfonic acid] (HEPES) and1,3-bis[tris(Hydroxymethyl)methylamino]propane (Bis Tris Propane). Inone embodiment the buffer is PBS, Tris or MOPS and in one embodiment thebuffer system is PBS, and more particularly 10×PBS. In accordance withone embodiment the 10×PBS buffer at pH 7.4 comprises the followingingredients:

1.37M NaCl

0.027M KCl

0.081M Na₂HPO₄

0.015M KH₂PO₄

5 mM MgCl₂

55.5 mM glucose

To create 10×PBS buffers of different pH, the ratio of Na₂HPO₄ andKH₂PO₄ is varied. Ionic strength may be adjusted as an independentvariable by varying the molarity of NaCl only.

The polymerization of the solubilized collagen composition is conductedat a pH selected from the range of about 6.0 to about 9.0, and in oneembodiment polymerization is conducted at a pH selected from the rangeof about 5.0 to about 11.0 and in one embodiment about 6.0 to about 9.0,and in one embodiment polymerization is conducted at a pH selected fromthe range of about 6.5 to about 8.5, in another embodimentpolymerization of the solubilized collagen composition is conducted at apH selected from the range of about 7.0 to about 8.0, and in anotherembodiment polymerization of the solubilized collagen composition isconducted at a pH selected from the range of about 7.3 to about 7.4.

The ionic strength of the buffered solution is also regulated. Inaccordance with one embodiment the ionic strength of the solubilizedcollagen composition is selected from a range of about 0.05 to about 1.5M, in another embodiment the ionic strength is selected from a range ofabout 0.10 to about 0.90 M, in another embodiment the ionic strength isselected from a range of about 0.14 to about 0.30 M and in anotherembodiment the ionic strength is selected from a range of about 0.14 toabout 0.17 M.

In still other illustrative embodiments, the polymerization is conductedat temperatures selected from the range of about 0° C. to about 60° C.In other embodiments, polymerization is conducted at temperatures above20° C., and typically the polymerization is conducted at a temperatureselected from the range of about 20° C. to about 40° C., and moretypically the temperature is selected from the range of about 30° C. toabout 40° C. In one embodiment the polymerization is conducted at about37° C.

In yet other embodiments, the phosphate concentration is varied. Forexample, in one embodiment, the phosphate concentration is selected froma range of about 0.005 M to about 0.5 M. In another illustrativeembodiment, the phosphate concentration is selected from a range ofabout 0.01 M to about 0.2 M. In another embodiment, the phosphateconcentration is selected from a range of about 0.01 M to about 0.1 M.In another illustrative embodiment, the phosphate concentration isselected from a range of about 0.01 M to about 0.03 M. In otherillustrative embodiments, the solubilized collagen composition can bepolymerized by, for example, dialysis against a solution under any ofthe above-described conditions (e.g., dialysis against PBS at pH 7.4),extrusion or co-extrusion of submucosa formulations into a desiredbuffer, including the buffers described above, or wet-spinning to formstrands of extracellular matrix material. In one embodiment the strandscan be formed by extrusion of a solubilized collagen composition througha needle and can be air-dried to form threads.

In one embodiment the strands can be formed by extrusion through aneedle and can be air-dried to form fibers or threads of variousdimensions. The syringe can be adapted with needles or tubing to controlthe dimensions (e.g., diameter) of the fibers or threads. In oneembodiment, the extrusion process involves polymerization of thesolubilized extracellular matrix composition followed by extrusion intoa bath containing water, a buffer, or an organic solvent (e.g.,ethanol). In another embodiment, the extrusion process involvescoextrusion of the solubilized extracellular matrix composition with apolymerization buffer (e.g., the buffer such as Tris or phosphatebuffers at various concentrations can be varied to control pH and ionicstrength). In yet another embodiment, the extrusion process involvesextrusion of the solubilized extracellular matrix composition into apolymerization bath (e.g., the buffer such as Tris or phosphate buffersat various concentrations can be varied to control pH and ionicstrength). The bath conditions affect polymerization time and propertiesof the fibers or threads, such as mechanical integrity of the fibers orthreads, fiber dimensions, and the like. In one embodiment the extrusionof a solubilized collagen composition through a needle is used a methodto control orientation of polymerized fibrils within the fibers. In oneembodiment, the fibers can be air-dried to create materials that can becrosslinked or woven into three dimensional meshes or mats that canserve as a substrate, or a component of a substrate, for culturingcells. In various illustrative embodiments, engineered extracellularmatrices can be polymerized from the solubilized extracellular matrixcomposition at any step in the above-described methods. For example, theengineered matrices can be polymerized from the solubilizedextracellular matrix composition after the solubilization step or afterthe separation step, the filtration step, or the lyophilization andrehydration steps, if the separation step, the filtration step, and/orthe lyophilization and rehydration steps are performed.

The engineered matrices can be combined, prior to, during, or afterpolymerization, with nutrients, including minerals, amino acids,pharmaceutical agents, sugars, peptides, proteins, vitamins (such asascorbic acid), or glycoproteins that facilitate cellular proliferation,such as laminin and fibronectin, or growth factors such as epidermalgrowth factor, platelet-derived growth factor, transforming growthfactor beta, or fibroblast growth factor, and glucocorticoids such asdexamethasone. In other illustrative embodiments, fibrillogenesismodulators, such as alcohols, glycerol, glucose, or polyhydroxylatedcompounds can be added prior to or during polymerization. In accordancewith one embodiment, cells can be added to the solubilized extracellularmatrix composition as the last step prior to the polymerization or afterpolymerization of the matrix. In another illustrative embodiment,particulate extracellular matrix compositions can be added to thesolubilized extracellular matrix composition and can enhance in vivobulking capacity. In other illustrative embodiments, cross-linkingagents, such as carbodiimides, aldehydes, lysl-oxidase,N-hydroxysuccinimide esters, imidoesters, hydrazides, and maleimides,and the like can be added before, during, or after polymerization.

Hyaluronic acid (HA) is a glycosaminoglycan found naturally within theextracellular matrix. This mucopolysaccharide is made up of a repetitivesequence of two modified simple sugars, glucuronic acid and N-acetylglucosamine. HA molecules are negatively charged and typically high inmolecular weight (long in size). The size and charged nature of thismolecule allow it to bind water to produce a high viscosity gel. Whenhyaluronic acid is added to soluble collagen compositions and thesolubilized collagen compositions are allowed to polymerize, it appearsthat only subtle changes occur to the fibrillar microstructure of theresultant 3D matrix. On the other hand, increasing the hyaluronic acidcontent significantly affects the viscous fluid phase of theextracellular matrix, providing it with distinct mechanical behavior.Furthermore, the addition of hyaluronic acid to engineered matrices wasfound to modulate the manner by which cells remodel and contract thematrix. Accordingly, HA content represents a further variable of thepresent engineered 3D matrices that can be manipulated to provide anoptimal microenvironment for cells cultured within the matrices.

In accordance with one embodiment the engineered purified collagen basedmatrices of the present invention can be used as cell culture substratesthat more accurately mimic the substrates that various cells contact invivo. Accordingly, collagenous based matrices can be designed forspecific cell types to mimic their native environment. In this mannerstem cells or progenitor cells can be cultured in vitro without alteringthe fundamental cell behavior (e.g., cell proliferation, growth,maturation, differentiation, migration, adhesion, gene expression,apoptosis and other cell behaviors) of the cells. In another embodiment,the engineered purified collagen based matrices of the present inventioncan be used to expand or differentiate a cell population, such a stemcell population (including pluripotent or unipotent cells), primarycells, progenitor cells or other eukaryotic cells by seeding the cellson, or within, the collagen based matrix and culturing the cells invitro for a predetermined length of time under conditions conducive forthat cell type's proliferation (i.e., appropriate nutrients,temperature, pH, etc.). In accordance with one embodiment cells areadded to the solubilized collagen composition as the last step prior tothe polymerization of the solubilized collagen composition. Theengineered purified collagen based matrices of the present invention canbe combined with nutrients, including minerals, pharmaceutical agents,amino acids, sugars, peptides, proteins, vitamins (such as ascorbicacid), or glycoproteins that facilitate cellular proliferation, such aslaminin and fibronectin and growth factors such as epidermal growthfactor, platelet-derived growth factor, transforming growth factor beta,or fibroblast growth factor, and glucocorticoids such as dexamethasone.

In one example of an embodiment comprising a collagen based matrixseeded with living cells, a sterilized engineered purified collagenbased matrix may be seeded with living cells and packaged in anappropriate medium for the cell type used. For example, a cell culturemedium comprising Dulbecco's Modified Eagles Medium (DMEM) can be usedwith standard additives such as non-essential amino acids, glucose,ascorbic acid, sodium pyruvate, fungicides, antibiotics, etc., inconcentrations deemed appropriate for cell type, shipping conditions,etc.

The cell seeded engineered purified collagen based matrices of thepresent invention can be used simply for culturing cells in vitro, orthe composition can be implanted or injected as a tissue graft constructto enhance the repair of damaged or diseased tissue. In one embodimentan improved tissue graft construct is provided wherein the constructcomprises a 3D purified collagen based matrix and a population of cells.The 3D purified collagen based matrix is formed from a solubilizedcollagen composition wherein the solubilized composition is formed bycontacting a source of purified collagen with an acid selected from thegroup consisting of hydrochloric acid, acetic acid, formic acid,sulfuric acid, ethanoic acid, carbonic acid, nitric acid, or phosphoricacid. The solubilized collagen composition is then polymerized asdescribed above to form the 3D purified collagen based matrix.

Cells, and in one embodiment stem cells, are combined with the collagenbased matrix at a low density and can be either added to the solubilizedcollagen composition prior to polymerization, or after formation of thecollagen based matrix. This initial seeded population of cells can beexpanded by incubating the composition under conditions suitable forreplication of the seeded cells. Accordingly, cell seeded 3D purifiedcollagen based matrices of the present invention comprise a populationof cells that consists of, or are the progeny of, eukaryotic stem cellsinitially added to the composition at a low density. In one embodiment atissue graft construct is prepared comprising the 3D purified collagenbased matrices of the present invention that have been seeded with a lowdensity of cells, wherein the cells are cultured within the matrix toexpand and/or differentiate the seeded population of cells prior toimplantation of the graft construct in a host. In one embodiment thecells, and more particularly stem cells, are initially seeded within the3D purified collagen matrix at a final concentration of about 10 toabout 10⁸ cells per milliliter, and in one embodiment at a finalconcentration of less than 10⁵ cells per milliliter.

For most cells, cell survival during in vitro culture is known todecrease as the concentration/density at which the cells are initiallyseeded onto a substrate. Applicants have discovered that using anengineered purified collagen based matrix and seeding stem cells at verylow densities, clonal populations of stem cells can be isolated in asubstantially pure form. Typically the isolation of non-embryoic stemcells results in the isolation of cells that may differentiate alongdifferent cell lineage pathways. In accordance with one embodiment ofthe present invention culturing conditions can be selected wherein adecreased seeding density of viable pluripotent or multipotent stemcells within an engineered purified collagen based matrix leads toclonal growth of cells representing a single cell lineage. Such cellscan be isolated and transferred to a second engineered purified collagenbased matrix and conditions can be altered to enhance the proliferationof the isolated clonal population of cells. Estimates of optimal celldensities for clonal growth range from about 10 cells/ml to about 10³cells/ml and depend upon the specific seeding efficiencies.

In accordance with one embodiment a method of isolating clonalpopulations of individual stem cells is provided. The method comprisesthe steps of contacting a source of collagen with hydrochloric acid toprepare a solubilized collagen composition. The solubilized collagencomposition is then polymerized to form an engineered purified collagenbased matrix. The stem cells are seeded on or within the engineeredpurified collagen based matrix at a low density that maintains thefunctionality of the stem cells but allows for the isolation of clonalpopulations of cells. In accordance with one embodiment the solubilizedcollagen composition is prepared having a type I collagen concentrationselected from the range of about 1.0 to 3.0 mg/ml, and a pH of about 6.5to about 7.0, wherein the solubilized collagen composition furthercomprises glucose and calcium chloride. In one embodiment stem cells areseeded at a concentration selected from the range of from about 10 toabout 10³ cells per milliliter. In one embodiment the source of collagenused to prepare the solubilized collagen composition comprises apurified preparation of type I collagen that has been dissolved in ahydrochloric acid solution. In an alternative embodiment the source ofcollagen comprises a hydrochloric acid solubilized fraction of anaturally occurring extracellular matrix, such as a submucosal matrix.In one embodiment the solubilized collagen composition is prepared fromvertebrate intestinal submucosa. The hydrochloric acid solution used toprepared the solubilized collagen composition can be from about 0.005 Nto about 0.1 N, from about 0.01 N to about 0.1 N, from about 0.05 N toabout 0.1 N, from about 0.001 N to about 0.05 N, about 0.001 N to about0.01 N, or from about 0.01 N to about 0.05 N HCl.

In any of the embodiments described in this application, the solubilizedcollagen composition (i.e., purified collagen or extracellular matrixcomponents) can be polymerized at final concentrations of collagen (dryweight/ml) of about 5 to about 10 mg/ml, about 5 to about 30 mg/ml,about 5 to about 50 mg/ml, about 5 to about 100 mg/ml, about 20 to about50 mg/ml, about 20 to about 60 mg/ml, or about 20 to about 100 mg/ml.Illustratively, the three-dimensional matrices may contain fibrils withspecific characteristics, including, but not limited to, a fibril areafraction (defined as the percent area of the total area occupied byfibrils in a cross-sectional surface of the matrix; i.e., fibrildensity) of about 7% to about 26%, about 20% to about 30%, about 20% toabout 50%, about 20% to about 70%, about 20% to about 100%, about 30% toabout 50%, about 30% to about 70%, or about 30% to about 100%. Infurther illustrative embodiments, the three-dimensional matrices have anelastic or linear modulus (defined by the slope of the linear region ofthe stress-strain curve obtained using conventional mechanical testingprotocols; i.e., stiffness) of about 0.5 kPa to about 40 kPa, about 30kPa to 100 kPa, about 30 kPa to about 1000 kPa, about 30 kPa to about10000 kPa, about 30 kPa to about 70000 kPa, about 100 kPa to 1000 kPa,about 100 kPa to about 10000 kPa, or about 100 kPa to about 70000 kPa.

In accordance with one embodiment a kit is provided for preparing 3Dmatrices that have been optimized for a particular cell that is to beseeded within the formed 3D matrix. The kit is provided with purifiedindividual components that can be combined to form a solubilizedcollagen composition that upon polymerization forms a 3D matrixcomprised of collagen fibrils that presents an optimal microenvironmentfor a population of cells. Typically the population of cells representcells provided separately from the kit, but in one embodiment the cellsmay also constitute a component of the kit. In one embodiment the cellsare mammalian cells, including human cells, and in a further embodimentthe cells are stem or progenitor cells. In accordance with oneembodiment a kit is provided comprising a solubilized collagencomposition and a polymerization composition. In a further embodimentthe solubilized collagen composition comprises purified type I collagenas the sole collagen component. In another embodiment the solubilizedcollagen composition comprises purified type I collagen and type IIIcollagen as the sole collagen components.

In one embodiment the kit comprises separate vessels, each containingone of the following components: purified type I collagen, a phosphatebuffer solution, a glucose solution, a calcium chloride solution and abasic neutralizing solution. In one embodiment the purified type Icollagen of the kit is provided in a lyophilized form and the kit isfurther provided with a solution of HCl (or other dilute acid includingfor example, acetic acid, formic acid, lactic acid, citric acid,sulfuric acid, ethanoic acid, carbonic acid, nitric acid, or phosphoricacid) for resuspending the lyophilized collagen. In one embodiment thekit is provided with a solution comprising a solubilized collagencomposition, and in a further embodiment the solubilized collagencomposition comprises a solubilized extracellular matrix composition. Inone embodiment the kit comprises a phosphate buffer solution, a glucosesolution, a calcium chloride solution, and acid solution, a basicneutralizing solution, a vessel comprising purified type I collagen, anda vessel comprising purified type III collagen. In one embodiment thepolymerization composition comprises a phosphate buffer that has a pH ofabout 7.2 to about 7.6 and the acid solution is an HCl solutioncomprising about 0.05N to about 0.005N HCl, and in one embodiment theacid solution is about 0.01N HCl. In one embodiment the glucose solutionhas a concentration selected from the range of about 0.2% to about 5%w/v glucose, or about 0.5% to about 3% w/v glucose, and in oneembodiment the glucose solution is about 1% w/v glucose. In oneembodiment the CaCl₂ solution has a concentration selected from therange of about 2 mM to about 40.0 mM CaCl₂ or about 0.2 mM to about 4.0mM CaCl₂, or about 0.2 to about 2 mM CaCl₂. In one embodiment the kit isprovided with a 10×PBS buffer having a pH of about pH 7.4, andcomprising about 1.37M NaCl, about 0.027M KCl, about 0.081M Na₂HPO₄,about 0.015M KH₂PO₄, about 5 mM MgCl₂ and about 1% w/v glucose.

The kit can further be provided with instructional materials describingmethods for mixing the kit reagents to prepare 3D matrices. Inparticular, the instructions materials provide information regarding thefinal concentrations and relative proportions of the matrix componentsthat give optimal microenvironmental conditions including fibrilmicrostructure and mechanical properties for a particular cell type orfor a particular desired result (i.e., clonal expansion of cells,differentiation, etc.).

The following examples illustrate specific embodiments in furtherdetail. These examples are provided for illustrative purposes only andshould not be construed as limiting the invention or the inventiveconcept in any way.

EXAMPLE 1 Preparation of Lyophilized, Bioactive ECM Compositions fromFractionated Submucosa Hydrolysates

Small intestinal submucosa is harvested and prepared from freshlyeuthanized pigs as previously disclosed in U.S. Pat. No. 4,956,178.Intestinal submucosa is powderized under liquid nitrogen and stored at−80° C. prior to use. Digestion and solubilization of the material isperformed by adding 5 grams of powdered tissue to each 100 ml ofsolution containing 0.1% w/v pepsin in 0.01 N hydrochloric acid andincubating for 72 hours at 4° C. Following the incubation period, theresulting solubilized composition is centrifuged at 12,000 rpm for 20minutes at 4° C. and the insoluble pellet is discarded. The supernatantis dialyzed against at least ten changes of 0.01 N hydrochloric acid at4° C. (MWCO 3500) over a period of at least four days. The solubilizedfractionated composition is then sterilized by dialyzing against 0.18%peracetic acid/4.8% ethyl alcohol for about two hours. Dialysis of thecomposition is continued for at least two more hours, with additionalchanges of sterile 0.01 N hydrochloric acid per day, to eliminate theperacetic acid. The contents of the dialysis bags are then lyophilizedto dryness and stored.

EXAMPLE 2 Preparation of Lyophilized, Bioactive ECM Compositions fromNon-Fractionated Submucosa Hydrolysates

Small intestinal submucosa was harvested and prepared from freshlyeuthanized pigs as previously disclosed in U.S. Pat. No. 4,956,178.Intestinal submucosa was powderized under liquid nitrogen and stored at−80° C. prior to use. Partial digestion of the material was performed byadding 5 g powdered tissue to each 100 ml solution containing 0.1% w/vpepsin in 0.01M hydrochloric acid and digesting for 72 hours at 4° C.Following partial digestion, the suspension was centrifuged at 12,000rpm for 20 minutes at 4° C. and the insoluble pellet discarded. Thesupernatant was lyophilized to dryness.

EXAMPLE 3 Preparation of Reconstituted, Bioactive ECM Compositions

Immediately prior to use, lyophilized material from Example 2,consisting of a mixture of extracellular matrix components, wasreconstituted in 0.01 N HCl. To polymerize the soluble extracellularmatrix components into a 3-dimensional matrix, reconstitutedextracellular matrix solutions were diluted and brought to a particularpH, ionic strength, and phosphate concentration by the addition of aphosphate buffer and concentrated HCl and NaOH solutions. Polymerizationof neutralized solutions was then induced by raising the temperaturefrom 4° C. to 37° C. Various polymerization buffers (including, e.g.,phosphate buffers) were used and the pH of the polymerization reactionwas controlled by varying the ratios of mono- and dibasic phosphatesalts. Ionic strength was varied based on sodium chloride concentration.

Type I collagen prepared from calf skin was obtained from Sigma-AldrichCorporation, St. Louis, Mo., and dissolved in and dialyzed extensivelyagainst 0.01 M hydrochloric acid (HCl) to achieve desiredconcentrations. Interstitial ECM was prepared from porcine smallintestinal submucosa (SIS). SIS was powdered under liquid nitrogen andthe powder stirred (5% w/v) into 0.01 N hydrochloric acid containing0.1% (w/v) pepsin for 72 h at 4° C. The suspension was centrifuged at12,000×g for 20 min at 4° C. to remove undissolved tissue particulateand lyophilized to dryness. Immediately prior to experimental use, thelyophilized material was redissolved in 0.01 N HCl to achieve desiredcollagen concentrations. To polymerize the soluble collagen orinterstitial ECM components into a 3D matrix, each solution was dilutedand brought to the specified pH, ionic strength, and phosphateconcentration by the addition of a polymerization composition andconcentrated HCl and NaOH solutions. Polymerization of neutralizedsolutions was induced by raising the temperature from 4° C. to 37° C.Various polymerization compositions were used to make final solutionswith the properties shown in Table 2.

TABLE 2 Collagen formulations SIS formulations pH I [P_(i)] [C] pH I[P_(i)] [C] Series 1 6.5 0.16 0.01 1 mg/ml 6.5 0.16 0.01 1 mg/ml 7.00.16 0.01 1 mg/ml 7.0 0.16 0.01 1 mg/ml 7.4 0.17 0.01 1 mg/ml 7.4 0.170.01 1 mg/ml 8.0 0.17 0.01 1 mg/ml 8.0 0.17 0.01 1 mg/ml 8.5 0.17 0.01 1mg/ml 8.5 0.17 0.01 1 mg/ml 9.0 0.17 0.01 1 mg/ml 9.0 0.17 0.01 1 mg/mlSeries 2 7.4 0.06 0.02 1 mg/ml 7.4 0.06 0.02 1 mg/ml 7.4 0.10 0.02 1mg/ml 7.4 0.30 0.02 1 mg/ml 7.4 0.15 0.02 1 mg/ml 7.4 0.60 0.02 1 mg/ml7.4 0.20 0.02 1 mg/ml 7.4 0.90 0.02 1 mg/ml 7.4 0.25 0.02 1 mg/ml 7.41.20 0.02 1 mg/ml 7.4 1.50 0.02 1 mg/ml Series 3 7.4 0.15 0.00 1 mg/ml7.4 0.3 0.00 1 mg/ml 7.4 0.15 0.01 1 mg/ml 7.4 0.3 0.02 1 mg/ml 7.4 0.150.02 1 mg/ml 7.4 0.3 0.04 1 mg/ml 7.4 0.15 0.03 1 mg/ml 7.4 0.3 0.06 1mg/ml 7.4 0.15 0.04 1 mg/ml 7.4 0.3 0.08 1 mg/ml 7.4 0.15 0.05 1 mg/ml7.4 0.3 0.11 1 mg/ml Table 2: Engineered ECMs representing purified typeI collagen or a complex mixture of interstitial ECM components (SIS)were prepared at varied pH (series 1), ionic strength (series 2), andphosphate concentration (series 3). [C] represents collagenconcentration in mg/ml, [P_(i)] represents phosphate concentration in M,and I represents ionic strength in M.

Representative data showing the results of varying the polymerizationtemperature, buffer system, pH (using either a phosphate or trisbuffer), ionic strength, phosphate concentration or concentration of ECMmaterial, on stiffness (elastic modulus) of the formed 3D matrix ispresented in FIGS. 1A-1G. In summary, as the polymerization temperatureis increased from 4° C. up to 37° C., the polymerization rate and thestiffness of the formed 3D matrix increases. The effect of a temperaturegradient profile on the microstructural composition of the 3D matrix wasalso investigated. Polymerizing the matrix using a temperature ramp fromabout 4° C. to 37° C. over 30 minutes was compared to matrices formedusing a step increase in temperature to 37° C. and incubated at thattemperature for 30 minutes. The data revealed that fibrils formed usinga temperature ramp are longer in length and have decreased fibrildensity compared to matrices formed using a single step increase intemperature. As the pH of the polymerizing composition is increased,from about 7.0 up to about pH 9.2, the polymerization rate and thestiffness of the formed 3D matrix increases. Buffer selection was foundto play a role in determining the mechanical properties of the 3Dmatrix, and more particularly tris based buffers reduced stiffness morethan phosphate based buffers. Regarding ionic strength, peak stiffnesscoincides with maximum polymerization time at an ionic strength of about0.3 M. As phosphate concentration is increased, stiffness decreases,however the concentration of phosphate in a 1×PBS solution does not havea substantial effect on stiffness. As collagen content is increased thestiffness of the matrix is increased.

EXAMPLE 4 Three-Dimensional Imaging of Engineered ECM's by ConfocalReflection Microscopy

Solutions of type I collagen or interstitial ECM components werepolymerized in a Lab-Tek chambered coverglass and imaged using a BioRadRadiance 2100 MP Rainbow confocaumultiphoton microscope using a 60×1.4NA oil immersion lens. Optical settings were established and optimizedfor matrices after polymerization was complete. Samples were illuminatedwith 488 nm laser light and the reflected light detected with aphotomultiplier tube (PMT) using a blue reflection filter. A z step of0.2 μm was used to optically section the samples. Because the resolutionof the z axis is less than that of the x-y plane, the sampling along thez axis may be different from that of the x-y. Images were collected inthe range of 10-25 μm from the upper surface of the coverglass.

EXAMPLE 5 Quantification of Fibril Properties from Three DimensionalImages

Quantification of the fibril diameter distribution within engineeredextracellular matrices was conducted based upon two- andthree-dimensional image sets obtained via electron and confocalmicroscopy techniques using methods described within Brightman et al.,Biopolymers 54:222-234, 2000. More recently, a Matlab program with agraphical user interface was written for measurement of fibril diametersfrom these images. For three-dimensional confocal images, depthattenuation was corrected by normalizing against a fitted logarithmiccurve, after which images were binarized into white and black pixelsusing a threshold value. Three rectangles were outlined in the x-y planeacross each fibril, with one axis aligned with the fibril. Averagefibril diameter in each rectangle was calculated as the total white areadivided by the rectangle's length. The average diameter of each fibrilwas taken to be the average of the three measurements, and the averagediameter in a given matrix was calculated as an average of allmeasurements.

Length of fibril per volume was estimated by dividing the total whitevolume of an image by the average cross-sectional area of fibrils inthat image. Due to distortion in the z-plane, the fibril cross-sectionsin the image could not be assumed circular and calculated from diameter.Rather, the average cross-sectional area was found by expanding therectangles described above into three-dimensional boxes. Thecross-sectional area of a fibril in was found by dividing the totalwhite volume contained in the box by the length of the box's axisaligned with the fibril.

A Matlab program has also been developed to determine fibril densityfrom two- and three-dimensional images. This method involvesthresholding and binarizing the image data to discriminate fibrils fromthe background. The surface area or volume representing fibrils is thenquantified and normalized to the surface area or volume of the image.

EXAMPLE 6 Spectrophotometry of Extracellular Matrix Polymerization

The time-course of polymerization was monitored in a Lambda 35 UV-VISspectrophotometer (Perkin-Elmer) equipped with a temperature-controlled,8-position cell changer as described previously by Brightman et al.,2000.

EXAMPLE 7 Rheometric Measurements of Extracellular Matrices

Mechanical properties of the matrices were measured using a TAInstruments AR-2000 rheometer. Neutralized collagen or SIS was placed onthe peltier temperature-controlled lower plate at 6° C., and the 40-mmparallel-plate geometry was lowered to a 1-mm gap. The temperature wasthen raised to 37° C. as oscillation measurements were made every 30seconds at 1 Hz and 5% strain. After polymerization was complete, anoscillation frequency sweep was made at 5% strain, from 0.1 to 3 Hz. Ashear creep test was then conducted with a shear stress of 1 Pa for 1000seconds.

EXAMPLE 8 Preparation of Reconstituted Bioactive Extracellular Matrices

Small intestinal submucosa was harvested and prepared from freshlyeuthanized pigs as previously disclosed in U.S. Pat. No. 4,956,178.Intestinal submucosa was powderized under liquid nitrogen and stored at−80° C. prior to use. Digestion and solubilization of the material wasperformed by adding 5 grams of powdered tissue to each 100 ml ofsolution containing 0.1% pepsin in 0.01 N hydrochloric acid andincubating with stirring for 72 hours at 4° C. Following the incubationperiod, the solubilized composition was centrifuged at 12,000 rpm for 20minutes at 4° C. and the insoluble pellet was discarded. The supernatantwas dialyzed extensively against 0.01 N HCl at 4° C. in dialysis tubingwith a 3500 MWCO (Spectrum Medical Industries). Polymerization of thesolubilized extracellular matrix composition was achieved by dialysisagainst PBS, pH 7.4, at 4° C. for about 48 hours. The polymerizedconstruct was then dialyzed against several changes of water at roomtemperature and was then lyophilized to dryness.

The polymerized construct had significant mechanical integrity and, uponrehydration, had tissue-like consistency and properties. In one assay,glycerol was added prior to polymerization by dialysis and matrices withincreased mechanical integrity and increased fibril length resulted.

EXAMPLE 9 Preparation of Extracellular Matrix Threads

Small intestinal submucosa was harvested and prepared from freshlyeuthanized pigs as previously disclosed in U.S. Pat. No. 4,956,178.Intestinal submucosa was powderized under liquid nitrogen and stored at−80° C. prior to use. Digestion and solubilization of the material wasperformed by adding 5 grams of powdered tissue to each 100 ml ofsolution containing 0.1% w/v pepsin in 0.01 N hydrochloric acid andincubating for 72 hours at 4° C. Following the incubation period, thesolubilized composition was centrifuged at 12,000 rpm for 20 minutes at4° C. and the insoluble pellet was discarded.

The solubilized extracellular matrix composition (at 4° C.) was placedin a syringe with a needle and was slowly injected into a PBS solutionat 40° C. The solubilized extracellular matrix composition immediatelyformed a filament with the diameter of the needle. If a blunt-tippedneedle is used, straight filaments can be formed while coiled filamentscan be formed with a bevel-tipped needle. Such filaments can be used asresorbable sutures.

EXAMPLE 10 Lyophilization and Reconstitution of SolubilizedExtracellular Matrix Compositions

Frozen small intestinal submucosa powder that had been prepared bycryogenic milling was centrifuged at 3000×g for 15 minutes and theexcess fluid was decanted. The powder (5% weight/volume) was digestedand solubilized in 0.01 N HCl containing 0.1% weight/volume pepsin forapproximately 72 hours at 4° C. The solubilized extracellular matrixcomposition was then centrifuged at 16,000×g for 30 minutes at 4° C. toremove the insoluble material. Aliquots of the solubilized extracellularmatrix composition were created and hydrochloric acid (12.1 N) was addedto create a range of concentrations from 0.01 to 0.5 N HCl.

Portions of the solubilized extracellular matrix composition weredialyzed (MWCO 3500) extensively against water and 0.01 M acetic acid todetermine the effects of these media on the lyophilization product.Aliquots of the solubilized extracellular matrix composition in 0.01 Macetic acid were created and glacial acetic acid (17.4 M) was added tocreate a range of concentrations from 0.01 to 0.5 M acetic acid. Thesolubilized extracellular matrix compositions were frozen using a dryice/ethanol bath and lyophilized to dryness. The lyophilizedpreparations were observed, weighed, and dissolved at 5 mg/ml in either0.01 N HCl or water. The dissolution and polymerization properties werethen evaluated. The results are shown in Tables 2-6.

TABLE 3 Gross appearance of solubilized extracellular matrixcompositions following lyophilization at various hydrochloric acidconcentrations. [HCl] (N) Appearance 0.01 Light, fluffy, homogenous,foam-like sheet; white to off-white in color; pliable 0.05 Slightlywrinkled and contracted, some inhomogeneities in appearance noted,slight brown tint, pliable to slightly friable in consistency 0.10Wrinkled, collapsed in appearance; inhomogeneities noted, some regional“melting” noted; significant brown tint; friable 0.25 Wrinkled,collapsed in appearance; increased inhomogeneities noted, increasedareas of regional “melting” noted; significant brown tint; friable 0.50Significant collapse and shrinkage of specimen, dark brown colorationthroughout; dark brown in color; friable

TABLE 4 Dissolution properties of solubilized extracellular matrixcompositions following lyophilization at various hydrochloric acidconcentrations. [HCl] (N) Reconstitution Reconstitution PropertiesMedium H₂O 0.01 N HCl 0.01 Completely dissolved in 20-30 Completelydissolved minutes, pH 4 in 20-30 minutes, pH 2 0.05 Majority dissolvedin 2 hours; Majority dissolved in slight particulate noted, pH 3-4 40minutes; very slight particulate noted, pH 2 0.1 Incomplete dissolutionIncomplete dissolution 0.25 Incomplete dissolution Incompletedissolution 0.50 Incomplete dissolution Incomplete dissolution

TABLE 5 Polymerization properties of solubilized extracellular matrixcompositions following lyophilization at various hydrochloric acidconcentrations. [HCl] (N) Reconstitution Polymerization PropertiesMedium H₂O 0.01 N HCl 0.01 Polymerized within 20-30 Polymerized withinminutes 10-20 minutes 0.05 Weak polymerization noted at Polymerizedwithin 45 minutes; significant lag time 20-30 minutes in polymerization0.1 *No Polymerization *No Polymerization 0.25 *No Polymerization *NoPolymerization 0.50 *No Polymerization *No Polymerization

TABLE 6 Dissolution properties of solubilized extracellular matrixcompositions following lyophilization at various acetic acidconcentrations. [Acetic Acid] Reconstitution Properties (M)Reconstitution in H₂O Reconstitution in 0.01 N HCl 0.01 Completelydissolved in 90 minutes, Completely dissolved in 90 pH 5 minutes, pH 1-20.05 Near complete dissolution after 90 Completely dissolved in 90minutes; small particulate remained, minutes, pH 1-2 pH 5 0.1 Completelydissolved in 90 minutes, Near complete dissolution in 90 pH 5 minutes;small particulate, pH 1-2 0.25 Completely dissolved in 90 minutes,Completely dissolved in 90 pH 5 minutes, pH 1-2 0.50 Near completedissolution after 90 Completely dissolved in 90 minutes; smallparticulate remained, minutes, pH 1-2 pH 5

TABLE 7 Polymerization properties of solubilized extracellular matrixcompositions following lyophilization at various acetic acidconcentrations. [Acetic Acid] (M) Reconstitution PolymerizationProperties Medium H₂O 0.01 N HCl 0.01 Polymerized within 5-10 minutesPolymerized within 5-10 minutes 0.05 Polymerized within 5-10 minutesPolymerized within 5-10 minutes 0.1 Polymerized within 5-10 minutesPolymerized within 5-10 minutes 0.25 Polymerized within 5-10 minutesPolymerized within 5-10 minutes 0.50 Polymerized within 5-10 minutesPolymerized within 5-10 minutes

These results show that lyophilization in HCl and reconstitution ofsolubilized extracellular matrix compositions in 0.01 N HCl to 0.05 NHCl or in water maintains the capacity of the components of thecompositions to polymerize. The results also show that lyophilization inacetic acid maintains the capacity of the components of the compositionsto polymerize when the composition is polymerized in water or HCl. Thesolubility rate is lyophilization from 0.01 N HCl>lyophilization from0.01 M acetic acid≧lyophilization from water.

EXAMPLE 11 Preparation of Solubilized Sis Composition This procedureoutlines a standard technique for the preparation of SIS solution. 1.Dissolution: of SIS Powder in Acetic Acid with Pepsin

-   -   1.1. Preparation of acetic acid with pepsin        -   1.1.1. Prepare the desired volume of 0.5 M acetic acid            (typically 1 L; this requires 28.7 mL of 17.4 M glacial            acetic acid).        -   1.1.2. Add the desired mass of pepsin to achieve a 0.1% w/v            solution (typically 1 g, if 1 L of acetic acid is used).        -   1.1.3. Place the jar containing acetic acid and pepsin on a            stir plate and begin mixing.    -   1.2. Preparation of centrifuged SIS powder        -   1.2.1. Place SIS powder in 50 mL centrifuge tubes.        -   1.2.2. Centrifuge SIS powder at 3000×g for 15 minutes.        -   1.2.3. Open centrifuge tubes, pour off and dispose of            supernate.        -   1.2.4. Remove pellets from tubes. Measure out the desired            mass to achieve a 5% w/v solution (typically 50 g, if 1 L of            acetic acid was used). Previously prepared and frozen            material may be used, and excess centrifuged material may be            frozen for later use.    -   1.3. Add centrifuged SIS pellet material to acetic acid/pepsin        solution. 1.4. Cover and allow it to stir for 72 hours at 4° C.

2. Centrifugation of Dissolved SIS

-   -   2.1. When removed from stirring, the SIS/pepsin solution should        appear viscous and somewhat uniform. Pour SIS/pepsin solution        into centrifuge jars. Balance jars as necessary.    -   2.2. This mixture should be centrifuged at 16,000×g for 30        minutes at 4° C. Refer to the operators manual or SOP for        instructions on using the centrifuge. If using the Beckman model        J2-21, use the JIO head at a speed of 9500 rpm.    -   2.3. Remove jars of SIS from centrifuge. Pour the supernate into        a clean jar. Be careful not to disturb the pellet, and stop        pouring if the SIS begins to appear more white and creamy (this        is pellet material).

3. Dialysis of SIS in Water and Hydrochloric Acid

-   -   3.1. Prepare dialysis tubing as follows:        -   3.1.1. Use dialysis tubing with MWCO 3500, diameter 29 1            mll. Handle dialysis tubing with gloves, and take care not            to allow it to contact foreign surfaces, as it may easily be            damaged.        -   3.1.2. Cut dialysis tubing to the necessary length.            (typically, 3 sections of about 45 cm).        -   3.1.3. Wet tubing in millipore water, and leave tubing in            the water until each piece is needed.        -   3.1.4. Do the following with each length of tubing:            -   3.1.4.1. Place a clip near one end of the tubing.            -   3.1.4.2. Holding the tubing to avoid contact with                foreign surfaces, use a pipette to fill the tubing with                SIS solution. Each piece of tubing should receive                roughly the same volume of SIS (for example, if three                lengths of tubing are used, measure one third of the                total volume into each).            -   3.1.4.3. Place a clip on the open end of the dialysis                tubing. Avoid leaving slack. The tube should be full and                taut.            -   3.1.4.4. Place the filled dialysis tubing in a container                of 0.01 M HCL with a stir bar.            -   3.1.4.5. Repeat the above steps to fill all lengths of                tubing.        -   3.1.5. Leave containers to stir at 4° C.    -   3.2. Details regarding changing the dialysis in 0.01 M HCl are        given below.        -   3.2.1. The 0.01 M HCl in the dialysis containers must be            changed several times. This should be done as follows:        -   3.2.2. After changing the 0.01 M HCl, another change should            not be done for at least two hours.        -   3.2.3. Change the 0.01 M HCl at least 10 times, over a            period of at least four days. This assumes a ratio of 200 mL            SIS to 6 L of 0.01 M HCl. If a higher ratio is used, more            changes may be necessary.        -   3.2.4. When changing 0.01 M HCl, do not leave dialysis bags            exposed in the air or on the counter. Use tongs or forceps            to move a dialysis bag directly from one container to            another. (It is okay to have multiple dialysis bags in one            container.) Dump the first container in the sink, then            refill it with millipore water. The dialysis bags can now be            placed in the newly filled container while the other            container or containers are changed.

4. Sterilization of SIS

-   -   4.1. Place dialysis bags of SIS in a solution of 0.18% Peracetic        acid/4.8% Ethanol. Leave to stir for two hours (more time may be        necessary).    -   4.2. Translocate dialysis bags to 0.01 M HCl, and continue        dialysis as before. Continue for at least 2 days, changing HCl        at least 3 times daily.    -   4.3. When dialysis is complete, dialysis tubing filled with SIS        should be removed from the HCl.    -   4.4. Remove the clips. Cut open one end of the dialysis tubing        and pour SIS into a clean jar.    -   4.5. SIS should be refrigerated until use.

5. Lyophilization of SIS

-   -   5.1. Operating the Vertis Freezemobile        -   5.1.1. Make sure the condenser is free of any water. (The            condenser is the metal cylinder which opens on the front of            the lyophilizer.) Ensure that the black rubber collection            tubing attached to the bottom of the condenser is plugged.            This can be accessed by opening the grate on the front of            the lyophilizer.        -   5.1.2. Close the door of the condenser, the top of the            manifold, and all sample ports. If the door of the condenser            or the top of the manifold are not forming a good seal apply            a small amount of vacuum grease to the rubber contact            surfaces.        -   5.1.3. Turn on the “Refrigerate” switch. The indicator on            the front of the lyophilizer will show a light beside            “Condenser” and beneath “On.” The light beneath “OK” will            not illuminate until the condenser is cooled. The condenser            temperature is indicated when the digital readout displays            “C1.”        -   5.1.4. When the “condenser” indicator light under “OK” is            illuminated, on the “Vacuum” switch. The indicator will show            a light beside “Condenser” and beneath “On.” The light            beneath “OK” will not turn on until the chamber is            sufficiently evacuated. The chamber pressure is indicated            when the digital readout displays “V 1.”        -   5.1.5. The rollers can be used for freezing a coat of            material on the inside surface of a jar. To use the rollers,            first ensure that the drain tube is plugged. (This can be            accessed through the door on the right side of the front of            the lyophilizer.) Using 100% Ethanol, fill the roller tank            to a level several millimeters above the top of the rollers.            Under-filling will cause ineffective cooling while            over-filling will allow ethanol to leak into the jars. The            temperature of ethanol bath is indicated when the digital            readout displays “T1.” This bath is cooled when the            “Refrigerate” switch is turned on. The “Rollers” switch            controls the turning of the rollers, and may be switched off            when no jar is on the rollers.    -   5.2. Lyophilizing SIS        -   5.2.1. Lyophilization jars, glass lids, and rubber gaskets            should be cleaned with ethanol. Allow ethanol to evaporate            completely before use. Mid-size jars, lids, and gaskets            (3-inch diameter) should be used to fit into the roller if            using the Virtis Freezemobile Jar lyophilization.        -   5.2.2. Pipette 75 mL of SIS solution into the lyophilization            jar. Place gasket and lid on jar.        -   5.2.3. Seal the jar by covering the openings with parafilm.            Note the small hole on the neck of the lid, which must be            covered.        -   5.2.4. Place the jar of SIS on the lyophilizer rollers for a            minimum of 2 hours.        -   5.2.5. Alternatively, the jar may be placed in a freezer            until all material is solid. In a −80° C. freezer, this            takes about 30 minutes.        -   5.2.6. Prepare a spigot on the lyophilizer by inserting a            glass cock with the tapered end out. The tapered end of the            cock should be coated with vacuum grease.        -   5.2.7. Remove the jar of SIS from the rollers (or freezer).            Place springs on the hooks to hold the jar and lid together.            Remove the parafilm and place the neck of the lid of the jar            over the cock. Rotate the jar so that the holes in the lid            and the cock do not align. The spigot can be rotated so that            the jar rests on the top surface of the lyophilizer.        -   5.2.8. Turn the valve switch so that it points toward the            jar of S1S.        -   5.2.9. More jars may be added to freeze-dry simultaneously,            but add jars one or two at a time. Wait until the vacuum            pressure falls to a reasonable range (e.g., 200 millitorr)            to ensure that the last jar is sealed before adding            subsequent jars.        -   5.2.10. Leave the jars under vacuum for at least 24 hours.        -   5.2.11. After lyophilization is complete, turn the switch On            the spigot to point away from the jar. This will allow air            into the jar.        -   5.2.12. Remove the jar from the cock.        -   5.2.13. Lyophilized material is not immediately used, it            should be stored in a dry environment. Use a large, sealable            container with Dri-Rite or another desiccant, and place            containers of lyophilized material therein.

6. Rehydration of Lyophilized SIS

-   -   6.1. Place lyophilized SIS into a tube or jar.    -   6.2. Add the desired quantity of liquid (typically 0.01 N HCl)        to the container of SIS.    -   6.3. Mixing may be accelerated by shaking, stirring, etc. Store        container under refrigeration until dissolution of SIS is        complete.

Sterilization of Solubilized SIS by Dialysis Against Peracetic AcidContaining Solution

1. Dialyze solubilized SIS against a large reservoir containing 0.18%peracetic acid/4.8% ethanol in water. Dialysis time may vary dependingupon peracetic acid concentration, dialysis membrane molecular weightcut off, temperature, etc.

2. Transfer dialysis bags aseptically to reservoirs containing 0.01 NHCl. Dialyze extensively to reduce concentration of residual peraceticacid.

3. When dialysis is complete, dialysis tubing filled with solubilizedSIS should be removed from the dialysis tank aseptically.

4. Remove dialysis clips and pour or pipette solubilized SIS into asterile jar.

5. The disinfected solubilized SIS should be stored at 4° C. until use.

Sterilization of SIS by Direct Addition of Peracetic Acid to SISSolution

1. Add 100% Ethanol and 32 wt % peracetic acid to solubilized SIS tocreate a solution with final concentration of 0.18% peracetic acid/4.8%ethanol. Stir well and leave for two hours.

2. Place solubilized SIS in aseptic dialysis bags. Dialyze againststerile solution of 0.01 N HCl.

3. When dialysis is complete, dialysis tubing filled with solubilizedSIS should be removed from the dialysis tank aseptically.

4. Remove dialysis clips and pour or pipette solubilized SIS into asterile jar.

5. The disinfected solubilized SIS should be stored at 4° C. until use.

EXAMPLE 12 Engineered ECM Compositions Regulate Cell Behavior

The three-dimensional (3D) extracellular matrix (ECM) of tissues in vivorepresents a complex array of macromolecules that serves to providebiochemical and biophysical microenvironmental cues to resident cells.However, the exact role of any one biophysical feature or molecularcomponent within the ECM in regulating cellular behavior has beendifficult to elucidate due to the inherent interdependence of ECMcompositional, structural, and mechanical properties. Recently,applicants have established that the 3D microstructural composition offibrils within engineered ECMs created from purified type I collagenregulates cell-matrix adhesion, matrix remodeling, and proliferationproperties of fibroblasts. It is further anticipated that altering theratios of collagen types I and III within engineered ECMs would affectthe hierarchical assembly of fibrils, and therefore the ECM signalingcapacity.

Engineered ECMs were created with altered ratios of collagen types IIIand I ranging from 1:6 to 1:2. Application of confocal and scanningelectron microscopy showed that ECMs prepared with increasing amounts oftype III collagen possessed an increasing number of small diameterfibrils. Furthermore, these microstructural changes translated intoalteration of matrix mechanical properties. Finally, results showed abiphasic response for fibroblast proliferation, morphology, and matrixremodeling.

EXAMPLE 13 Engineered ECM Compositions Regulate Stem CellDifferentiation

A multipotential mesenchymal stem cell line (D1) derived from mousebone-marrow stroma was obtained from American Type Culture Collection(ATCC). D1 cells were propagated in Dulbecco's modified Eagle mediumcontaining 4.5 g/L glucose, 110 mg/L sodium pyruvate, 100 U/mlpenicillin, 100 μg/ml streptomycin, and 10% fetal bovine serum (FBS)within a humidified atmosphere of 5% carbon dioxide at 37° C.Three-dimensional collagen ECMs were prepared by dissolving native,acid-solubilized type I collagen from calf skin (Sigma Chemical Co, St.Louis, Mo.) in 0.01 N hydrochloric acid to achieve desiredconcentrations. As a final purification step the isolated collagenobtained from Sigma Chemical was dialyzed against an acidic solutionhaving a low ionic strength (0.01 N HCl) for 1-2 days, using dialysistubing or a membrane having a molecular weight cut-off selected from arange of about 3,500 to about 12,000 daltons. For sterile preparationsof collagen, the purified collagen solution was layered onto a volume ofchloroform. After incubation for 18 hours at 4° C., the collagensolution layer was carefully removed so as not to include thecollagen-chloroform interface layer.

To produce 3D purified collagen matrices with microstructures of variedcollagen fibril dimensions (e.g., length, diameter, density), collagensolutions were polymerized under different conditions. Specifically, tocreate collagen matrices consisting of collagen fibrils at increasingdensities, collagen solutions were polymerized at final collagenconcentrations of 1.0 to 3.0 mg/ml. The polymerization compositioncomprised a 10× phosphate buffered saline (PBS) with an ionic strengthof 0.14 N and a pH of 7.4. The specific formulation of the 10× phosphatebuffer is as follows:

10×PBS, pH 7.4

1.37M NaCl

0.027M KCl

0.081M Na₂HPO₄

0.015M KH₂PO₄

5 mM MgCl₂

1% w/v glucose

To create 10×PBS buffers of different pH, the ratio of Na₂HPO₄ andKH₂PO₄ is varied. Ionic strength can be adjusted as an independentvariable by varying the molarity of NaCl only. To create the 3D matrixcomprising cells suspended within 3D matrix microenvironment thefollowing components were mixed together:

1 ml solubilized collagen (e.g., type I collagen) in 0.01N HCl

150 ul 10×PBS, pH 7.4

150 ul 0.1N NaOH

100 ul 13.57 mM CaCl₂

100 ul 0.01 N HCl

Final Volume 1.5 ml.

The composition is mixed well after each additional component is added.The composition is then combined with a cell pellet of known cell numberto create desired cell density; mixed well; and allowed to polymerize.The resulting polymerized 3D matrix has a final concentration of glucoseand CaCl₂ of about 5.55 mM glucose and about 0.9046 mM CaCl₂.

To create collagen matrices consisting of collagen fibrils that variedin length and width, collagen solutions were polymerized at a pHselected from the range of 6.5-8.5. D1 cells were harvested in completemedium, collected by centrifugation, and added as the last componentbefore polymerization. Tissue constructs were prepared at a relativelylow cell density of 5×10⁴ cells/ml. Previous studies by applicants haveshown that this cell density is suitable for maintaining cell viability,minimizing cell-cell interaction, and allowing the study of the dynamicrelationship between an individual cell and its surrounding ECM.

Polymerization of tissue constructs was conducted in 24-well cultureplates maintained in a humidified environment at 37° C. Immediatelyafter polymerization (20 minutes or less), complete medium was added andthe tissue constructs were cultured for 48 hours at 37° C. in ahumidified environment consisting of 5% CO₂ in air. After 48 hours, eachof the constructs comprising D1 cells seeded within a specific ECMmicrostructure were cultured under 3 different conditions:

1) complete medium no supplements

2) complete medium plus 10⁻⁷ M dexamethasone

3) complete medium plus 50 μg/ml ascorbic acid

For comparison purposes, parallel experiments were conducted on D1 cellsgrown in a standard 2D format on tissue-culture plastic. Cell behaviorand morphology were monitored throughout the duration of the experimentusing standard brightfield microscopy. After 24 days in culture, tissueconstructs were histochemically stained with alcian blue, oil red O, andalizirin red as indicators of chondrogenesis, adipogenesis, andosteogenesis.

Results:

The results of this experiment revealed the following:

1) multipotential stem cells seeded within engineered ECMs proliferatedat rates that were dependent upon microstructural composition of theengineered ECM and the media composition;

2) time-dependent patterns of cellular condensation and aggregationexhibited by multipotential stem cells were dependent uponmicrostructural composition of the engineered ECM and the mediacomposition;

3) time-dependent differentiation of multipotential stem cells seededwithin engineered ECMs was dependent upon microstructural composition ofthe engineered ECM and the media composition;

4) maintenance of precursor or multipotential cells in anundifferentiated state in vitro was dependent upon microstructuralcomposition of the engineered ECMs and the media composition;

5) patterns of cellular proliferation/differentiation for cells grownwithin 3D were different from those observed for cells grown in 2D ontissue culture plastic; and

6) Decreasing the cell density of viable multipotential stem cellswithin engineered ECMs led to clonal growth of a large population ofcells representing a single cell lineage. Estimates of optimal celldensities for clonal growth range from about 10 cells/ml to about 10³cells/ml and depend upon the specific seeding efficiencies. For mostcells, cell survival is known to decrease with seeding density.

EXAMPLE 14 Engineered ECM Compositions Regulate Unipotential Stem CellDifferentiation

A unipotential stem (precursor) cell line (L1) derived from mouse andrepresenting pre-adipocytes was obtained from American Type CultureCollection (ATCC). L1 cells were propagated in Dulbecco's modified Eaglemedium containing 4.5 g/L glucose, 110 mg/L sodium pyruvate, 100 U/mlpenicillin, 100 μg/ml streptomycin, and 10% fetal bovine serum (FBS)within a humidified atmosphere of 5% carbon dioxide at 37° C. To enhancecell viability, cells representing passage numbers greater than 5 weremaintained in complete media in which the penicillin and streptomycinwere reduced to 25 U/ml and 25 μg/ml, respectively.

Preparation of tissue constructs representing L1 cells seeded within 3Dengineered ECMs of different microstructural compositions was carriedout as described in Example 13. Immediately after polymerization (20minutes or less), complete medium was added and the tissue constructswere cultured for 48 hours at 37° C. in a humidified environmentconsisting of 5% CO₂ in air. After 48 hours, each of the constructscomprising L1 cells seeded within a specific ECM microstructure werecultured under 3 different conditions:

1) complete medium no supplements; medium changed every 2 daysthereafter;

2) complete medium no supplements and post differentiation mediumtreatment every 2 days thereafter; and

3) differentiation medium and post differentiation medium treatmentevery 2 days thereafter

The differentiation medium consists of DMEM supplemented with 10% FBS,25 U/ml penicillin, 25 μg/ml streptomycin, 115 μg/ml methyl-isobutylxanthine, 10 μg/ml insulin, and 5×10⁻⁷M dexamethasone. The postdifferentiation medium consisted of DMEM supplemented with 10% FBS, 25U/ml penicillin, 25 μg/ml streptomycin, and 10 μg/ml insulin. Forcomparison purposes, parallel experiments were conducted on L1 cellsgrown in a standard 2D format on tissue-culture plastic. Cell behaviorand morphology were monitored throughout the duration of the experimentusing standard brightfield microscopy.

Results:

The results of this experiment revealed the following:

1) unipotential stem (precursor) cells seeded within engineered ECMsproliferated at rates that were dependent upon microstructuralcomposition of the engineered ECM and the media composition;

2) time-dependent patterns of cellular condensation and aggregationexhibited by unipotential stem cells were dependent upon microstructuralcomposition of the engineered ECM and the media composition;

3) time-dependent differentiation of unipotential stem cells into matureadipocytes seeded within engineered ECMs was dependent uponmicrostructural composition of the engineered ECM and the mediacomposition;

4) maintenance of precursor cells in an undifferentiated state in vitrowas dependent upon microstructural composition of the engineered ECMsand the media composition; and

5) patterns of cellular proliferation/differentiation for cells grownwithin 3D were different from those observed for cells grown in 2D ontissue culture plastic.

EXAMPLE 15 Effect of Fibril Microstructure and Mechanical Properties of3D ECM on Cultured Stem Cells

Multi-potential stem cells derived from the bone marrow of mice (D1s;ATCC) were suspended at 5×10⁴ cells/ml within purified type I collagensolutions (Sigma Chemical Co.) at varying collagen concentrationsranging from 1.5-3.6 mg/ml using the procedures described in Example 13.Tissue constructs consisting of D1 cells entrapped within a 3D ECM wereformed by inducing self-assembly (polymerization) at pH 7.4, 137 mMNaCl, and 37° C. For this specific example, an increase in collagenconcentration as a self-assembly parameter, was used to generate a 3DECM microenvironment in which the density of the resultant fibrils andstiffness (linear or elastic modulus) of the matrix were systematicallyincreased. The 3D constructs and resident cells were maintained in oneof three different media formulations (Table 8) at 37° C. in ahumidified environment consisting of 5% CO₂ in oxygen for periods oftime up to 4 weeks. Basal medium consisted of Dulbecco's modifiedEagle's medium supplemented with 4 mM L-glutamine, 4.5 g/L glucose, 1.5g/L sodium bicarbonate, 1 mM sodium pyruvate, 10% fetal bovine serum,100 U/ml penicillin, and 100 μg/ml streptomycin. For comparisonpurposes, D1 cells also were cultured in a parallel fashion in thestandard 2D format on the surface of tissue culture plastic.

TABLE 8 Medium formulations used to culture D1 cells Medium DesignationMedium Formulation A Basal medium supplemented with 1 μM dexamethasone,0.5 mM isobutylmethylxanthine, 1 μg/ml insulin B Basal mediumsupplemented with 0.1 μM dexamethasone, 8 μg/ml ascorbic acid, 5 mMβ-glycerophosphate C Basal medium with no additives

After various periods of time, the proliferative and differentiationstatus of the cells were determined qualitatively or quantitatively.Qualitative evaluation of cell number and morphology was conductedseveral times a week using light microscopy. Real-time RT-PCR was usedto quantify and compare the expression levels of CFBA1 (runx2), LPL(lipoprotein lipase), and procollagen II as indicators of osteogenesis(bone formation), adipogenesis (fat formation), and chondrogenesis(cartilage formation), respectively. Histochemical stains, includingalkaline phosphatase and oil red O, were applied to whole mount orcryosectioned samples to detect osteogenic and adipogenic activity,respectively. In some cases immunohistochemical staining was used tocorroborate results.

Cells grown in basal culture medium with no additives (mediumformulation C) on standard tissue culture plastic (A) and within 3D ECMmicroenvironments of controlled fibril density and stiffness showeddistinct growth patterns and morphologies. Results showed as the fibrildensity and stiffness of the 3D ECM microenvironment increased, theproliferative capacity of the cells decreased. The dependence of D1proliferation on the stiffness of the 3D ECM microenvironment was notedfor all media formulations studied. More specifically, D1 cells grown onplastic or within the low stiffness 3D ECM microenvironment showed anincreased number of spindle-shaped cells. Within 2 weeks the cells onplastic reached confluence and formed a sheet of cuboidal shaped cells.On the other hand, spindle-shaped cells were evident within the lowstiffness ECM even after 4 weeks of culture. These cells appeared toremain undifferentiated and populated the ECM uniformly. Growth patternsindicative of isolated clonogenic events were higher in frequency withinECMs of increased stiffness.

The observed differences in the growth patterns and morphologies adoptedby cells grown in the 2D and 3D microenvironments suggested that themulti-potential cells were being directed down distinct differentiationpatterns. Limited directed differentiation appeared to occur for cellsgrown on plastic or within the low stiffness ECMs (1.5 mg/ml).Interestingly, D1 cells grown within 3D ECMs of high stiffness (3.4mg/ml) formed regional aggregates of cells indicative of osteogenesisand/or skeletal myogenesis. Osteogenesis but not myogenesis events werealso observed with engineered ECMs of moderate stiffness (3 mg/ml). Thebiochemical composition of the media also could be varied to enhance thedifferentiation of cells down a specific pathway or to maintain cells ina relatively undifferentiated state. Specifically, cells grown in mediumformulation A demonstrated a high frequency of adipogenesis on plasticand within 3D ECMs of low fibril density and stiffness (1.5 mg/ml). Asthe fibril density and stiffness of the 3D ECM microenvironmentincreased, adipogenesis events decreased and osteogenesis increased.Medium formulation B appeared to support differentiation of D1 cellsinto fat (adipogenesis) and (bone) osteogenesis on plastic. Limitedareas of osteogenesis and adipogenesis were noted amongst a large numberof spindle-shaped cells for D1 cells grown within ECMs of low stiffnessunder these same medium conditions. As the stiffness of the 3D ECMincreased, cells more uniformly developed regional areas of osteogenesisand myogenesis-like events. A 2D projection of one confocal imagerevealed cells organized or fused to form a multi-cellular structurereminiscent of a myotube. These events were limited to 3D ECMmicroenvironments of high stiffness (3.4 mg/ml and greater). While thesemyotube-like events were noted in all three medium formulations, theyappeared to occur more frequently in medium formulations B and C. Thecells of the myotube-like structure were stained immunohistochemicallyfor F-actin to demonstrate the fusion of and connectivity of the actincytoskeleton between individual cells.

Real-time RT-PCR confirmed that biophysical features of the 3D ECMmicroenvironment (e.g., fibril density and ECM stiffness) could bemodulated to regulate stem cell growth and differentiation. FIG. 8 showsthe differences in gene expression patterns for D1 cells grown for twoweeks on tissue culture plastic (Plastic) and within 3D engineered ECMsprepared at low (1.5 mg/ml), moderate (3.0 mg/ml), and high fibrildensity and stiffness (3.6 mg/ml). Again, cells subjected to each ofthese 2D and 3D culture formats were maintained in one of threedifferent media formulations (Table 8).

The tissue specific genes CBFA1 (runx2), LPL (lipoprotein lipase), andPro Col II (procollagen II) were selected as indicators of osteogenesis,adipogenesis, and chondrogenesis, respectively. Results showed thatcells grown for 2 weeks on 2D plastic in the basal medium (no additives)remain relatively undifferentiated, more specifically, limitedexpression of the osteogenic, adipogenic, and chondrogenic indicators.On the other hand, D1 cells show an increase in LPL (adipogenesis) whencultured on plastic in the presence of Medium A or Medium B. Theexpression of LPL correlates well with the observed fat cell morphologydeveloped within the cultures. Interestingly, the gene expressionpatterns developed by cells grown within a 3D ECM microenvironment weredramatically different from those observed for cells grown on plastic.Specifically, the expression of CBFA1, indicative of osteogenesis, couldbe enhanced by growing the cells within 3D ECMs of increased stiffnessor Medium B. Again, the increased expression of CFBA1 correlated wellwith cell morphologies and histochemical staining. Interestingly,chondrogenesis events as indicated by high procollagen II expressionappeared to be enhanced within D1 cells cultured in 3D ECMs of highstiffness.

The starting cell density was also a critical determinant of the stemcell fate within the 3D culture formats studied. Clonal growth and celldifferentiation events were favored by increasing the ECM stiffnessand/or by decreasing the starting cell density within a given 3D ECMformat. Adipogenesis was favored by decreasing the ECM stiffness and/orby increasing the cell density. Interestingly, adipogenesis was observedwithin high stiffness 3D ECMs only when the cell seeding densityapproached 1×10⁶ cells/ml and above.

EXAMPLE 16 Cell Culture

Low passage neonatal human dermal fibroblasts (NHDFs) were obtained fromCambrex Bioproducts (Walkersville, Md.). NHDFs were propagated infibroblast basal medium supplemented with human recombinant fibroblastgrowth factor, insulin, gentamicin, amphotericin-B, and fetal bovineserum (FBS) according to manufacturer's recommendations. Cells weregrown and maintained in a humidified atmosphere of 5% CO₂ at 37° C.Cells representing a limited passage number of 20 or less were used forall experiments.

EXAMPLE 17 Preparation of 3D Engineered ECMs and 3D Tissue Constructs

Purified type I and type III collagens, that were solubilized frombovine dermis and human placenta, respectively, were obtained from SigmaChemical Company (St. Louis, Mo.). Three-dimensional engineered ECMswere prepared at a constant collagen type I concentration of 1.5 mg/mland type III collagen concentrations of either 0, 0.25, 0.50, and 0.75mg/ml (Table 9) using the general procedures described in Example 13.The polymerization buffer consisted of 10× phosphate buffered saline(PBS) with an ionic strength of 0.14 M and a pH of 7.4. All 3Dengineered ECMs and tissue constructs were polymerized in vitro within ahumidified environment at 37° C. To determine the cellular signalingcapacity of each 3D ECM microenvironment, 3D tissue constructs wereformed by first harvesting NHDFs in complete media and then adding thecells as the last component to the collagen solutions prior topolymerization. Tissue constructs were prepared at a relatively low celldensity of 5×10⁴ cells/ml in order to minimize cell-cell interactions.Immediately after polymerization (20 minutes or less), complete mediumwas added and the tissue constructs were maintained at 37° C. in ahumidified environment consisting of 5% CO₂ in air.

TABLE 9 Summary of formulations for 3D engineered ECMs prepared withvaried ratio of collagen types I and III. Type I Type III Total CollagenType III Collagen collagen Collagen Type I/III Collagen (mg/ml) (mg/ml)(mg/ml) Ratio (% of Total) 1.5 0 1.50 0 0 1.5 0.25 1.75 6:1 14.3% 1.50.50 2.00 3:1 25.0% 1.5 0.75 2.25 2:1 33.3%

A summary of the results of the data generated by the experiment ofExample 13-17 is provided in FIG. 9.

EXAMPLE 18 Preparation of Two-Dimensional (2D) ECM Surface Coatings

To prepare 2D surfaces coated with the different ECM compositions,solutions containing collagen type I (1.5 mg/ml) and varyingconcentrations of collagen type III (0, 0.25, 0.5, and 0.75 mg/ml) werealiquoted (300 μl/well) into tissue culture plates (24-well) andair-dried within a laminar flow hood for approximately 18 hours. Wellplates containing 2D ECM surface coatings were equilibrated with PBS, pH7.4, prior to seeding the NHDFs at a density of 2.5×10⁴ cells/well.Complete medium was added and the NHDFs on the surface of the 2D ECMcoatings were maintained at 37° C. in a humidified environmentconsisting of 5% CO₂ in air.

EXAMPLE 17 Qualitative and Quantitative Analysis of 3D ECMMicrostructural Composition

Two quantitative parameters describing the 3D ECM microstructuralcomposition, fibril area fraction (a 2D approximation of 3D fibrildensity) and fibril diameter, were determined based upon confocalreflection and scanning electron microscopy (SEM) images. Prior tomicrostructural analysis, engineered 3D ECM constructs were polymerizedwithin four-well Lab-Tek coverglass chambers (Nalge Nunc International,Rochester, N.Y.) and placed within a humidified environment at 37° C.where they were maintained for approximately 15 hours. For measurementsof fibril area fraction, the confocal microscope was used to obtain highresolution, 3D, reflection images of the component collagen fibrilswithin each ECM (Brightman at al., Biopolymers 54: 222-234, 2000;Voytik-Harbin et al., Methods Cell Biol 63: 583-597, 2001). Three images(at least 10 μm in thickness) were taken at random locations within eachof 2 specimens representing a given 3D ECM composition. The confocalimage stacks were then read into Matlab (The Mathworks, Natick, Mass.),and 2D projections, representing 21 z-sections, of each image werecreated and a threshold chosen for binarization. Using a built-infunction in Matlab, the area occupied by collagen fibrils (white pixels)was calculated, converted to μm² based upon the pixel sizes, andnormalized to the total image area.

Fibril diameter measurements were made by applying Imaris 4.0 (BitplaneInc., Saint Paul, Minn.) to both confocal reflection and SEM images ofengineered ECM constructs. For SEM imaging, engineered ECM constructswere fixed in 3% glutaraldehyde in 0.1M cacodylate at pH 7.4, dehydratedwith ethanol, and critical point dried. Samples were sputter-coated withgold/palladium prior to imaging. Duplicate samples were imaged in a JEOL(Peabody, Mass.) JSM-840 SEM using 5 kV accelerating voltage and amagnification of 3,000×. Digital images were captured using 1280×960resolution and 160 second dwell time. From each image obtained fromduplicate samples, forty fibrils were chosen at random (10 fibrils perquadrant). Five lines were drawn perpendicular to the long axis of eachfibril using the measurement tool in Imaris (Brightman at al.,Biopolymers 54: 222-234, 2000). The average number of pixelsrepresenting the fibril diameter was then converted into μm based uponthe known pixel size.

EXAMPLE 18 Measurement of Tensile Mechanical Properties of 3D EngineeredECMs

Specimens for mechanical testing were prepared by polymerizing eachsoluble ECM formulation in a “dog bone” shaped mold as describedpreviously (Roeder et al., J Biomech Eng, 124: 214-222, 2002). In brief,the mold consisted of a glass slide and a piece of flexible siliconegasket. The gauge section of the mold measured 10 mm long, 4 mm wide,and approximately 1.5 mm thick. Neutralized ECM solution was added toeach mold and allowed to polymerize at 37° C. in a humidifiedenvironment where they were maintained for 18-20 hours prior to tensileloading. Polypropylene mesh was embedded in the ends of each 3D ECMconstruct to facilitate clamping for mechanical loading.Low-magnification, 4D images (x, y, z, and time) of each ECM constructduring uniaxial tensile loading were acquired using an integratedmechanical loading-stereomicroscope set-up. This set-up involvedinterfacing a modified (Roeder et al., J Biomech Eng, 124: 214-222,2002.) Minimat 2000 miniature materials tester (Rheometric Scientific,Inc., Piscataway, N.J.) with a Stemi 2000-C Stereomicroscope (Carl ZeissMicroImaging; Thornwood, NY) mounted with a DFC480 high-resolution colordigital camera (Leica Microsystems, Cambridge, UK). Strategically placedright-angle prisms (Edmund Industrial Optics, Barrington, N.H.) wereused to monitor changes in specimen thickness (z-direction) throughoutthe loading process. The image field was positioned to include the clampthat was attached to the load cell in order to provide a “fixed” frameof reference throughout the loading process. Each ECM construct wasloaded uniaxially at an extension rate of 1 mm/min (corresponding to astrain rate of {dot over (ε)}≈0.04/min) until failure. Images werecollected at a rate of 0.1 frames/sec to provide sequential images at0.64% strain intervals. Changes in the width (x-direction) and thickness(z-direction) dimensions of the specimen's gauge section were measureddirectly from low-magnification digital camera images representing thewidth and thickness of the specimen and used to calculatecross-sectional area. The mechanical behavior of each specimen,including engineering stress (σ_(e)), true stress (σ_(t)), and appliedstrain (ε_(ap)) were calculated from load-displacement recordingsprovided by the Mini-mat. Applied strain was calculated by simplifyingthe Lagrangian strain definition (Malvern, Introduction to the Mechanicsof a Continuous Medium. Upper Saddle River, N.J.: Prentice-Hall, 1969)for a simple stretch λ (new length divided by original length) asindicated below

$\begin{matrix}{ɛ_{ap} = {\frac{1}{2}( {\lambda^{2} - 1} )}} & (1)\end{matrix}$

Engineering stress was calculated as

$\begin{matrix}{\sigma_{e} = \frac{F}{A_{o}}} & (2)\end{matrix}$

where, F was the force recorded by the Minimat and A_(o) was the initialcross-sectional area (width×thickness) within the center of the specimen(Callister et al., Materials Science and Engineering: An Introduction.3rd edition. New York, N.Y.: John Wiley & Sons, 1994). For calculationof true stress, the actual cross-sectional area of each specimen at aspecific load was imaged, quantified, and substituted for A_(o) in theengineering stress equation above. From the resulting stress-strainrelationships ultimate strength (maximum stress achieved during tensileloading), failure strain (strain at which specimen fails), and linear orelastic modulus (stiffness; slope of linear region of stress-straincurve) were determined.

EXAMPLE 19 Multi-Dimensional Confocal Imaging of Cell-ECM Interactions

All multi-dimensional imaging was performed on a Bio-Rad Radiance 2100MP Rainbow (Bio-Rad, Hemel Hempstead, England) multi-photon/confocalsystem adapted to a TE2000 (Nikon, Tokyo, Japan) inverted microscopewith a heated stage set at 37° C. (ALA Scientific Instruments, Westbury,N.Y.). A custom-designed environmental chamber was adapted to themicroscope to provide tissue constructs with a sterile environment of 5%CO₂ in humidified air (Pizzo et al., J Appl Physiol 98: 1909-1921,2005). For each of the engineered ECMs studied, at least 5 individualcells were repeatedly monitored during the first 5 hours followingconstruct polymerization. During the collection of time-lapse images,the confocal microscope was used in a reflection (back-scattered light)mode to obtain image stacks of an individual cell and the componentcollagen fibrils of its surrounding ECM as described previously(Brightman et al., Biopolymers 54: 222-234, 2000; Voytik-Harbin et al.,Methods Cell Biol 63: 583-597, 2001). Images were collected at 30-minuteintervals and a z-step of 0.5 μm to minimize exposure of the tissueconstructs to radiation from the confocal microscope laser.

EXAMPLE 20 3D Cell Morphometric Analysis

Three-dimensional confocal images used for qualitative and quantitativeanalyses of NHDF morphology were collected using the confocal microscopein a combination reflection-epifluorescence mode (Voytik-Harbin et al.,Methods Cell Biol 63: 583-597, 2001; Voytik-Harbin et al., MicroscMicroanal 9: 74-85, 2003). Immediately following the 6-hr time-lapseimaging, tissue constructs were stained with the vital dye, Cell TrackerGreen (Molecular Probes, Eugene, Oreg.), to facilitate discrimination ofthe cell from the surrounding collagen ECM. The processed image stackwas used to determine fundamental morphological parameters includingnumber of cytoplasmic projections, cell volume, 3D cell surface area,length, width, and height as described previously (Pizzo et al., J ApplPhysiol 98: 1909-1921, 2005). Since each cell had a relatively uniqueorientation within the 3D matrix, these morphological parameters weredefined based on a cellular coordinate system. Morphological evaluationwas conducted on a total of 10 to 23 cells for each of the 3D ECMcompositions studied.

EXAMPLE 21 Determination of 3D Average Local Principal Strains andPoints of Maximum Local Principal Strain

Consecutive time-lapse confocal reflection images representing thetime-dependent deformation to the collagen fibril microstructure inducedby an individual resident cell provided the basis for quantification oflocal ECM remodeling in terms of 3D displacements and strains. Strainswere quantified using an incremental digital volume correlation (IDVC)algorithm developed previously by our laboratory (Roeder et al., JBiomech Eng 126: 699-708, 2004). To determine 3D average local principalstrains within the surrounding ECM induced by an individual cell, firstthe 15×15×3 grid of displacements (174.2×174.2×24 μm³ total volume) fromthe IDVC algorithm were converted into 3D strains with x, y, and zdirections based on the confocal coordinate system. The strains in theentire volume were then averaged in each of the three confocaldirections to give a 3×3 symmetric matrix of average strains, ε_(avg),such that

$\begin{matrix}{ɛ_{avg} = \begin{bmatrix}ɛ_{xx} & ɛ_{xy} & ɛ_{xz} \\ɛ_{xy} & ɛ_{yy} & ɛ_{yz} \\ɛ_{xz} & ɛ_{yz} & ɛ_{zz}\end{bmatrix}} & (3)\end{matrix}$

where ε_(ij) are average strains in the confocal coordinate systemdirections. This average strain matrix in Equation (3) was then solvedusing eigenvector analysis (Strang et al., Linear Algebra and ItsApplications. 3rd edition. San Diego, Calif.: Academic Press, 1988) todetermine 3 average principal strains (E₁, E₂, E₃) and associateddirections such that,

$\begin{matrix}{{ɛ_{avg} \cdot \lbrack V\rbrack} = {\lbrack V\rbrack \cdot \begin{bmatrix}E_{1} & 0 & 0 \\0 & E_{2} & 0 \\0 & 0 & E_{3}\end{bmatrix}}} & (4)\end{matrix}$

where [V] is a 3×3 matrix such that the column vectors (V₁, V₂, V₃) arethe directions of the principal strains given by

[V]=[V₁ V₂ V₃]  (5)

Therefore, the deformation induced by each cell had a unique set ofaverage principal strains and directions in 3D.

Another analysis was performed to determine on a finer scale where themaximum local principal strains within the 3D ECM occurred inrelationship to the cell. This analysis involved determination of localprincipal strains E₁, E₂, and E₃, each with unique principal direction,at each of the 15×15×3 grid points. The maximum compressive E₁, E₂, andE₃ were then identified within the image volume. The location of eachmaximum compressive principal strain was known in terms of its IDVC gridlocation and also in μm. The distance from these three-maximum principalstrain locations to the center of the cell body in 3D could then bedetermined using simple vector relationships. The locations of themaximum compressive principal strains did not necessarily occur at thesame grid locations for each cell.

EXAMPLE 22 Labeling and Visualization of Actin Cytoskeleton within 3DEngineered Tissue Constructs

Tissue constructs formed by seeding NHDFs within specific 3D ECMformulations during polymerization were prepared in four-well Lab-Tekcoverglass chambers (Nalge Nunc International, Rochester, N.Y.) forvisualization of the F-actin cytoskeleton. At specified timepoints,constructs were fixed and permeabilized with a solution containing 0.1%Triton 100× and 3% paraformaldehyde, post-fixed in 3% paraformaldehyde,and treated with 1% bovine serum albumin to minimize non-specificbinding. The constructs were then stained overnight at 4° C. with AlexaFluor 488 Phalloidin (Molecular Probes, Eugene, Oreg.) and rinsed.Three-dimensional images of the F-actin distribution within anindividual cell as well as its surrounding ECM were collectedsimultaneously using confocal microscopy in a combined epifluorescenceand reflection mode. When necessary, images were deconvolved usingAutoDeblur (Autoquant Imaging, Inc., Watervliet, N.Y.).

EXAMPLE 23 Qualitative and Quantitative Determination of CellProliferation

Quantification of NHDF proliferation and its dependency on the 3D ECMmicroenvironment involved preparing 3D tissue constructs within 24-welltissue-culture plates using an alamarBlue-based proliferation assay asdescribed previously (Pizzo et al., J Appl Physiol 98: 1909-1921, 2005;Voytik-Harbin et al., In Vitro Cell Dev Biol Anim 34: 239-246, 1998).For comparison purposes, the proliferative capacity of NHDF was alsodetermined for an equivalent number of cells seeded directly onto theplastic surface of a well-plate as well as 2D plastic surfaces coatedwith different ECM compositions consisting of type I collagen in thepresence of varying amounts of type III collagen. At time pointsrepresenting 24 and 48 hours after construct polymerization and/or cellseeding, each well and tissue construct was examined microscopically toobserve the viability, number, and morphology of the cells. The mediumfrom each well then was replaced with fresh medium containing themetabolic indicator dye alamarBlue (10% v/v; BioSource International,Inc., Camarillo, Calif.). Twenty-four hours later, dye reduction wasmonitored spectrofluorometrically using a FluoroCount MicroplateFluorometer (Packard Instruments, Meriden, Conn.) with excitation andemission wavelengths of 560 nm and 590 nm, respectively. Backgroundfluorescence measurements were determined from wells containing only dyereagent in culture medium. Maximum levels of relative fluorescence weredetermined from alamarBlue solutions that were autoclaved to inducecomplete dye reduction. The mean and the standard deviation values forall fluorescence measurements were calculated and subsequentlynormalized with respect to the background and maximum fluorescencereadings. All experiments were performed in triplicate and repeated atleast three times. When relevant, statistical analyses were performedusing Matlab and included an analysis of variance (ANOVA). TheTukey-Kramer method for multiple comparisons (p<0.05) was then applied.The two-tailed t-test (α=0.05) was applied for pairwise comparisons.

EXAMPLE 24 Three-Dimensional Microstructural Composition of EngineeredECMs Depends upon Collagen Type I/III Ratio

This study utilized the application of confocal microscopy in areflection mode and SEM facilitated microstructural analysis from both2D and 3D perspectives as well as at two different limits of resolution.SEM provided high-resolution (approximately 10 nm) 2D images of ECMmicroarchitecture after specimens had been critical point dried. On theother hand, confocal reflection microscopy allowed visualization of the3D microstructural organization of component collagen fibrils withinengineered ECMs in their fully hydrated or native state; however theresolution obtained with confocal imaging is approximately 200 nm,twenty times less than that obtained with SEM.

Both confocal and SEM images showed that ECMs prepared with increasedamounts of type III collagen possessed an increased number or density ofcollagen fibrils. The fibril area fraction (FIG. 2A) was quantified fromconfocal reflection images and showed a nearly linear increase with typeIII collagen over the range studied. Engineered ECMs prepared from typeI collagen alone had a fibril area fraction of 12.0±1.4% compared to21.5±2.6% for those formed in the presence of the highest concentration(0.75 mg/ml) of type III collagen. In addition to this effect on fibrildensity, increased levels of type III collagen resulted in a downwardshift in the fibril diameter distribution (Table 10) and (FIG. 2B). Themean fibril diameter as determined from SEM images for ECMs preparedwith type I collagen alone was 115.2±23.2 μm. The mean fibril diametershowed a significant (p<0.05) decrease to 94.8±23.0 μm and 87.0±17.0 μmfor ECMs in which collagen III was added at levels of 0.25 mg/ml and0.75 mg/ml, respectively. In general, fibril diameter measurements madefrom confocal reflection images corroborated SEM results; however,fibril diameter values obtained from confocal images were greater thanthose obtained using SEM (Table 10) since confocal imaging was conductedon unprocessed, fully hydrated specimens. It should be noted that fibrildiameter measurements made using confocal reflection imaging wereconsidered somewhat less accurate and less precise since fibrildiameters were near the limit of resolution for this imaging technique.

TABLE 10 Collagen fibril diameter measurement data for 3D engineeredECMs prepared from type I collagen in the absence and presence of typeIII collagen as determined from scanning electron (SEM) and confocalreflection (CRM) images. Fibril Type I Collagen Type I Collagen (1.5mg/ml) + Diameter (1.5 mg/ml) Type III Collagen (0.75 mg/ml) (nm) SEMCRM SEM CRM Mean ± 115.16 ± 412.63 ± 76.35 87.04 ± 17.00 384.60 ± 71.96SD 23.18 Median 112 408 86 378 Range 78-194 200-664 56-176 236-628

EXAMPLE 25 Mechanical Properties of Engineered ECMs Depend upon CollagenType I/III Ratio

Previously, we showed that engineered ECMs prepared at increasingconcentrations of type I collagen featured an increase in fibril densitybut no significant change in fibril diameter (Roeder et al., J BiomechEng, 124: 214-222, 2002). Furthermore, this change in ECMmicrostructure, specifically an increase in collagen fibril density, wasfound to be positively correlated with ECM tensile strength andstiffness (linear or elastic modulus (Roeder et al., J Biomech Eng, 124:214-222, 2002)). Traditionally, mechanical properties for collagen-basedmatrices have been calculated based upon engineering stress (Osborne etal., Med Biol Eng Comput 36: 129-134, 1998; Ozerdem et al., J BiomechEng 117: 397-401, 1995; Roeder et al., J Biomech Eng, 124: 214-222,2002), which assumes no change in specimen cross-sectional area duringmechanical loading. However, since it is known that our engineered ECMsexhibit Poisson's ratios on the order of 2 to 4 (Roeder et al., JBiomech Eng, 124: 214-222, 2002; Voytik-Harbin et al., Microsc Microanal9: 74-85, 2003), true stress was calculated to account for thesignificant reduction in cross-sectional area experienced by thescaffolds during testing. Since our experimental set-up facilitated thecontinuous monitoring of changes in specimen cross-sectional area duringtensile loading, true stress calculated parameters were considered tomost accurately reflect mechanical behavior of the ECMs.

ECMs engineered from type I collagen in the presence of type IIIcollagen over the range of 0 to 0.75 mg/ml (type III collagen content of0 to 33.3%) showed biphasic responses in terms of true stress calculatedparameters ultimate strength and stiffness. The mean ultimate strengthobtained for ECMs prepared from 1.5 mg/ml type I collagen alone was136.7±49.9 kPa. Addition of collagen III resulted in significantreductions in ultimate strength, with 66.7±4.2 kPa (p=0.0016) and75.1±22.7 kPa (p=0.0085) values being measured for ECMs prepared atcollagen III levels of 0.25 mg/ml and 0.75 mg/ml, respectively. Thelinear or elastic modulus (stiffness), as determined from the linearregion of the stress-strain curve, also showed reductions of 32%(p=0.0002) at the 0.25 mg/ml collagen III level and 18% (p=0.189) at the0.75 mg/ml collagen III level compared to those where no collagen IIIwas added. A decline in failure strain with increasing type III collagencontent was noted. Specifically, failure strain values decreasedsignificantly from 62.2±12.2% when no collagen III was added to53.3±1.4% (p=0.048) and 43.0±5.9% (p=0.002) when the type III collagencontent was 0.25 mg/ml and 0.75 mg/ml, respectively. Finally, increasingthe type I collagen content from 1.5 to 3 mg/ml increased ECM ultimatestrength and stiffness, confirming previous findings (Roeder et al., JBiomech Eng, 124: 214-222, 2002). ECMs prepared at 3 mg/ml type Icollagen had ultimate strength and stiffness values that were 2.2 and3.5 times, respectively, those obtained for ECM prepared at 1.5 mg/mltype I collagen.

EXAMPLE 26 3D Cell Morphology Depends Upon Collagen Type I/III Ratio

The ability of cells to sense and respond to changes in the 3D ECMmicroenvironment that resulted from the addition of type III collageninitially was assessed by determining and comparing 3D cell morphologyand cell-induced ECM remodeling (deformation and reorganization ofcomponent collagen fibrils). Three-dimensional morphometric analyses forcells seeded within the different ECM microenvironments were conductedat 6 and 12 hours following tissue construct formation. ECM remodelingby individual cells was repeatedly monitored during a 5 to 6 hour timewindow shortly after construct formation.

Notable differences in 3D cell morphometric parameters were detected atboth 6 and 12 hours as the cells probed and adapted to theirextracellular microenvironment.

One of the more prominent differences noted at 6 hours was that cellsseeded within engineered ECMs prepared at the lowest type III collagencontent (0.25 mg/ml) took on a rounded morphology with multiple shortprojections. This cell morphology contrasted that observed within ECMsprepared with 0.5 mg/ml and 0.75 mg/ml type III collagen. At thesehigher collagen III levels a large percentage of cells took on a morespindle-shaped cell body with fewer but prominent lengthy projections.Cells seeded within ECMs prepared from type I collagen alone took on amore spindle, bipolar shape and possessed the fewest (on average 2 to 4)but longest projections at the 6-hour time point.

By 12 hours, the morphological differences that resulted from thevarious ECM microenvironments were subtler, largely owing to varyinglevels of ECM remodeling induced by cells at this time. At 12 hours,cells seeded within all ECM formulations appeared relatively spindle,bipolar-shaped. However, qualitative and morphometric analyses indicatedthat as the type III collagen content increased within the ECM, cellsshowed a statistically significant decrease in length (p<0.05), a subtleincrease in width, and a decline in the length-to-width ratio asindicated (FIG. 3A-C). Based upon both qualitative and quantitative 3Dmorphology data, it appeared that cells grown in ECMs containing typeIII collagen took on a more contracted cell state. Despite the observedchanges in cell shape, no significant differences were noted in 3D cellsurface area (FIG. 3D) or cell volume at either time point. Consistentwith previous studies (Pizzo et al., J Appl Physiol 98: 1909-1921,2005), a larger proportion of cells grown within ECMs of higher totalcollagen content possessed an increased number of cytoplasmicprojections at both 6- and 12-hour time points; however, this effect onprojection number was less obvious when the collagen content was alteredby adding type III collagen rather than type I collagen. Collectivelythese results demonstrate that cells adapt their shape, including thenumber and length of their projections, in response to ECMs that vary incollagen type I/III ratio. Furthermore, the morphological differencesbetween cells appeared to be related to stiffness properties of the ECM.

EXAMPLE 27 Collagen Type I/III Ratio of 3D Microenvironment AffectsContractile State of the Cell and ECM Remodeling

The collagen type I/III ratio also affected the ability of individualcells to deform and reorganize the component collagen fibrils of theirsurrounding ECM. Repeated monitoring of interactions between a cell andits surrounding collagen fibrils within a live tissue construct byconfocal reflection microscopy provided a means of visualizing andquantifying this response over a 5 to 6 hour time window. An IDVCalgorithm (Roeder et al., J Biomech Eng 126: 699-708, 2004) was appliedto consecutive confocal image stacks and used to determine 3Ddisplacements and strains as they occurred locally to a given cell andadjacent collagen fibrils. Data generated from this algorithm providedthe basis for 1) quantification of volumetric strain induced by a singlecell within a tissue construct; 2) a detailed analysis of average localprincipal strains for each imaged volume; and 3) determination ofmagnitudes and locations for points within the image volume wheremaximum principal strains, E₁, E₂, and E₃, occurred. This data was thencompiled and used to compare the mechanical status of a large number ofindividual cells grown within the different ECM formulations.

Results showed that cells grown in ECMs containing type III collagenwere less able to contract and remodel the surrounding matrix as thetype III collagen content increased or type I/III ratio decreased.Qualitative perspectives and corresponding volumetric strain dataobtained for representative cells grown within type I collagen ECMsprepared with low (0.25 mg/ml) and high (0.75 mg/ml) type III collagenconcentrations are shown (FIG. 4). Comparison of average local principalstrains induced by cells grown within the different ECM formulationsindicated that cells grown at low type III collagen levels (0.25 mg/ml)induced higher strain (approximately 3 to 3.5 greater) in each of thethree principal directions compared to those grown at high type IIIcollagen levels (0.75 mg/ml) and these differences were significant forE2 and E3 (p<0.05; FIG. 4). However, it is important to note that typeIII collagen containing ECMs with total collagen contents of 1.75 mg/mlto 2.25 mg/ml were characterized by 3D average local principal strainlevels that were about 2 to 3 times greater than ECMs prepared of type Icollagen alone and a total collagen content of 1 mg/ml. Analysis of thelocations and magnitudes for points of maximum principal strain in the1-, 2-, and 3-direction revealed that cells grown within engineered ECMsof type III collagen content of 0.25 mg/ml induced strain values thatwere approximately twice that exerted by cells grown within ECMscontaining 0.75 mg/ml type III collagen (FIG. 5). Furthermore, ingeneral, points of maximum principal strain for all three directionsoccurred at distances further from the center of the cell (FIG. 6) whengrown in ECMs at the low versus high type III collagen content.Specifically, maximum principal strains were observed at distances of40-50 μm from the center of the cell for ECMs containing 0.25 mg/ml typeIII collagen. However, cells within ECMs prepared with a type IIIcollagen content of 0.75 mg/ml generated maximum principal strains atdistances of only 15-25 μm from the center of the cell. Although theaddition of type III collagen enabled cells to induce large principalstrains within their ECMs, the distance at which maximum principalstrain occurred was considerably less than that found for ECMs preparedat low levels of type I collagen alone (FIG. 6). More specifically, ECMsprepared at 1 mg/ml type I collagen yielded, on average, points ofmaximum principal strain for 1-, 2-, and 3-directions at distances of 48μm, 45 μm, and 52 μm from the center of the cell, respectively. It wasalso noted that the locations of the maximum principal strain were oftenassociated with and occurred along major cell projections, especiallyfor cells grown at the high collagen type I/III ratios. Furthermore,fibril deformation patterns were dependent upon the collagen type I/IIIratio. Remodeling of ECMs containing type III collagen was characterizedby fibril condensation around the cell periphery. On the other hand,ECMs prepared from type I collagen alone showed regional areas of fibrilalignment. The difference in ECM remodeling, as indicated by bothqualitative fibril deformation and quantified strains suggesteddifferences in mechanical properties between fibrils formed fromhomotypic type I and heterotypic type I/III fibrils.

Based collectively on the observed effects of type III collagen on ECMmicrostructure/mechanical properties as well as differences in the 3Dcell morphology and cell-induced ECM remodeling within these matrices,it was hypothesized that varying the collagen type I/III ratio withinthe ECM microenvironment modulated the contractile state of residentcells. To test this hypothesis, cells were seeded within the 3D ECMmicroenvironments and the organization of cytoskeletal actin wasvisualized using confocal microscopy 6 hours post polymerization.Results showed prominent actin stress fiber formation for cells withinECMs containing collagen III. Well-organized actin bundles (stressfibers) were even observed within ECMs containing the highest collagenIII concentration, 0.75 mg/ml, despite the high total collagen contentand fibril density. Cells containing a few scattered actin filamentswere observed in ECMs prepared from type I collagen alone, but only atlow collagen concentrations of 1.5 mg/ml and below. Cells with diffuseactin staining patterns were noted within ECMs prepared at collagen Ilevels greater than 1.5 mg/ml. Diffuse actin staining patterns wereobserved for cells grown in engineered ECMs representing type I collagenECMs prepared at concentrations of 1 mg/ml and 3 mg/ml. A few organizedactin bundles were noted in engineered ECMs created from 1 mg/ml type Icollagen and a large number of organized actin bundles running parallelalong the major cytoplasmic projections or long axis of cells grownwithin engineered type I collagen ECMs formed in the presence of 0.75mg/ml type III collagen. Collectively, results showed that stress fiberformation, which is indicative of the contractile state of the cell, waspositively related to cell-induced local ECM remodeling and strain andinversely related to ECM stiffness.

EXAMPLE 28 Collagen Type I/III Ratio within 3D Engineered ECMs but not2D ECM Surface Coatings Modulates Cellular Proliferation

To determine the effect of the collagen type I/III ratio on thefundamental proliferative behavior of cells, NHDFs were seeded withinthe different ECM formulations. For comparison purposes, parallelstudies were conducted in which fibroblasts were seeded onto tissueculture plastic. The number of living cells present at 24 and 48 hoursfollowing cell seeding was quantified indirectly using the metabolicindicator dye alamarBlue and confirmed qualitatively. Consistent withprevious studies (Pizzo et al., J Appl Physiol 98: 1909-1921, 2005),cells grown within a 3D ECM microenvironment proliferated at decreasedrates compared to those grown in a 2D format on tissue culture plastic(FIG. 7A). Fibroblast proliferation was enhanced in ECMs with increasedtype III collagen content (FIG. 7A). Since the type I collagen contentwas kept constant, increasing the amount of type III collagen alsoincreased the overall collagen content. Although the total number ofcells within all ECM formulations increased between 24 and 48 hours, thetotal number of fibroblasts was greatest for ECMs prepared with thehighest type III collagen concentration for both time points. When typeIII collagen was added at levels below 0.25 mg/ml, in the range of 0.02mg/ml to 0.10 mg/ml, the proliferative capacity of the resident cellswas lower than that obtained for 1.5 mg/ml type I collagen alone.

Since the addition of type III collagen affected not onlymicrostructural-mechanical properties but also the macromolecularcomposition of the engineered ECMs, it was uncertain if changes in NHDFproliferation were a result of differences in biophysical or biochemicalsignals (cues) inherent in the 3D ECM microenvironments. To isolate thebiochemical and biophysical variables, traditional experimental methodsinvolving creation of 2D ECM surface coatings consisting of variedcollagen I/III ratios to evaluate cell-ECM interactions were applied.NHDF were seeded onto the ECM-coated surfaces and proliferationmonitored. No significant difference was observed in cell proliferationdue to type III collagen content at either the 24- or 48-hour timepoints (FIG. 7B). All coatings showed a significant increase (p<0.05) incell number between the 24- and 48-hour time points. And at the 48-hourtime point, cells seeded on plastic showed significantly greaterproliferation than those seeded on any of the ECM coated surfaces(p<0.05).

EXAMPLE 29 Comparison of Structure-Function of Engineered ECMFormulations

Various engineered ECM formulations were compared to analyzethree-dimensional microstructure-mechanical properties, including fibrilarea fraction, fibril diameter, and stiffness of the engineered ECM(Table 11). The various engineered ECM formulations were also comparedin regards to ECM contraction, morphology, and cell proliferation (Table11).

TABLE 11 Comparison of Structure-Function of Engineered ECM Formulations1.5 mg/ml Type I + Engineered ECM 1.0 mg/ml 1.5 mg/ml 3 mg/ml 0.75 mg/mlFormulation Type I Type I Type I Type III 3D ECMMicrostructure-Mechanical Properties Fibril Area + ++ +++ +++ Fraction(Density) Fibril Diameter ++ ++ ++ + Stiffness + ++ +++ + CellularResponse: ECM Contraction/Morphology/Proliferation ECM Contraction +++++ + ++/ +++ Distance +++ +++ ++ + Number of + ++ +++ + projectionsLength of Medium- Medium- Medium- Short projections Long Long LongMorphology Long- Long- Stellate Short- Spindle Spindle SpindleCytoskeletal Stress-fibers Stress-fibers Diffuse Stress-fibers ActinProliferation ++ ++ + +++

EXAMPLE 30

Recent studies have demonstrated that human adipose-derived stem cells(ASC) derived from adult human adipose tissue secrete bioactive levelsof multiple angiogenic and antiapoptotic growth factors includinggranulocyte-macrophage colony stimulating factor (GM-CSF), VEGF,hepatocyte growth factor (HGF), bFGF, and transforming growth factor-β(TGF-β), and are able to enhance blood flow and minimize death ofischemic muscle tissue [Rehman et al., 2004, Circulation 109: 1292-8].These results are important because they indicate that autologousdelivery of ASC, which are readily available from liposuction underlocal anesthesia, may be a novel and uniquely feasible therapeuticoption to enhance angiogenesis and tissue rescue in ischemia. However,quantitative analysis of cell delivery has documented that the majorityof peripheral blood mononuclear cells or ASC injected viaintramyocardial, intracoronary, and interstitial retrograde coronaryvenous (IRV) in an ischemic swine model are not retained in the heartimmediately following delivery and that the processes of delivery werehighly inconsistent. In addition, examination of ASC surviving 1 weekfollowing intramuscular injection showed reduction of cell numbers to25% of the injected cells over this period, suggesting limited cellsurvival [Rehman et al., 2004, Circulation 109: 1292-8]; this is furthercorroborated in the myocardial system by survival of approximately 20%or less of initially retained mesenchymal stem cells over 4 weekspost-injection.

The survival, proliferation, and differentiation properties of human APCand EPC cells implanted within three dimensional matrices will beinvestigated using both standard cell culture media or by suspension inany of the formulations of “ready-to-assemble” components ofself-assembling 3D matrix microenvironments, in which themicrostructure, composition, and mechanical properties are quantifiedand systematically varied. The delivery efficiency and subsequentengraftment (cell survival and differentiation) of human ASC orendothelial progenitor cells derived from human cord blood (EPC)implanted within an animal model of hindlimb muscle ischemia will alsobe investigated. More particularly, cells will be delivered with orwithout injectable 3D matrix microenvironments in which the“instructive” or signaling properties are controlled and systematicallyvaried.

Methods:

A series of in vitro experiments will be conducted to determine theeffect of specific biophysical features of a cell's 3D ECMmicroenvironment on the fundamental behavior of human adipose-derivedstem cells (ASC) and highly proliferative endothelial progenitor cellsderived from human cord blood (EPC). ASC will be harvested from humanadipose tissue as described previously [Rehman et al., 2004, Circulation109: 1292-8]. Cultures of endothelial progenitor cells will be obtainedfrom umbilical veins using established procedures [Ingram et al., 2004,Blood 104:2752-2760]. 3D ECM microenvironments in which specificbiophysical features including fibril density, length, and width andstiffness are systematically varied will be created from purifiedcollagen as described previously [Pizzo et al., 2005, J. App. Phys.98:1909-1921; Roeder et al., 2002 J. Biomech Eng. 124: 214-22213,17]. Inaddition, 3D microenvironments in which composition is systematicallyvaried by including ECM molecules such as type III collagen, hyaluronicacid, VEGF, bFGF will also be investigated. These molecules were chosenbased upon their known role in neovascularization and cardiac muscledevelopment. In all cases, cells will be added as the last component ofthe solubilized collagen matrix and the suspension will be injected over30 seconds through a 25 Ga needle (paralleling intramuscular injectionfor in vivo systems) into a well plate and polymerized at 37° C.Immediately following polymerization (less than 30 minutes), completemedium will be added to all constructs.

For these studies, cell seeding densities ranging between 1×10⁵ to 1×10⁷cells/ml will be evaluated. Fundamental cell behaviors includingsurvival, morphology, proliferation, and differentiation will bedetermined using techniques established previously [Pizzo et al., 2005,J. App. Phys. 98:1909-1921]. In some cases, cells will be prelabeledwith CellTracker dyes or transfected with GFP and analyzed in 3- or4-dimensions using confocal microscopy in a combinationreflection-fluorescence mode. Outcomes will be compared to those fromcontrol “deliveries” in which cells are injected into media withinculture plates, parallel to the situation for cell injection into atissue environment in the absence of a solubilized, self-assemblingmatrix.

In addition to the in vitro culturing of cells within the 3Dmicroenvironments, the 3D cell containing matrices will be implanted viainjection into either normal or ischemic muscle in vivo, using thehindlimb model of muscle ischemia that the March lab has established andpublished in the preliminary findings concerning adipose stem cells[Rehman et al., 2004, Circulation 109: 1292-8]. Briefly, nude mice areemployed so that cells of human origin can be studied in the absence ofxenogeneic barriers. The ilio-femoral artery is surgically ligated andexcised as described previously, in the left hindlimb only. The righthindlimb thus serves as a non-ischemic control. The musculature of thedistal legs (e.g., tibialis anterior) then can be used as awell-demarcated delivery site for 100 μl injections into normal (right)and ischemic (left) muscle, that are performed under directvisualization. Injections of precisely defined numbers of ASC or EPCwill be conducted 1 day following the surgical induction of ischemia inmice, with groups of 5 animals for each condition to be evaluated. Theconditions will include control injections in saline (the previousstandard) or in soluble self-assembling matrices. The cells will belabeled with GFP to permit enumeration by subsequent flow cytometryfollowing muscle dissociation, as well as microscopic evaluation of theanatomy of engraftment and differentiation in selected mice. Miceinjected will be sacrificed at either 3 hours post-injection, toquantify the number of cells retained acutely following delivery; and at2 weeks post-injection, to determine precisely the cell survival overtime following the injection. Cells will be counted by flow cytometrywith the addition of fluorescent particles to permit precise volumetricenumeration. A total of 60 mice will be used in this study (e.g., 2 celltypes×3 ECMs×5 animals/group×2 timepoints). The key endpoints will bequantitation of cell retention, and subsequent survival and engraftmentinto muscle or vasculature in the normal and ischemic muscles.

EXAMPLE 31 Effect of Hyaluronic Acid Content in 3D Matrices on CellBehavior Materials and Methods Cell Culture.

Low passage neonatal human dermal fibroblasts (NHDFs), growth media, andpassing solutions were obtained from Cambrex Bioproducts (Walkersville,Md.). NHDF were propagated in fibroblast basal medium supplemented withhuman recombinant fibroblast growth factor, insulin, gentamicin,amphotericin B, and FBS according to manufacturer's recommendation.Cells were maintained in a humidified atmosphere of 5% CO₂ at 37° C. andcell passage numbers representing 15 or less were used for allexperiments.

Engineered 3D Tissue Constructs.

To investigate the effect of hyaluronic acid (HA) on ECM assembly andsignaling, type I collagen matrices with varied HA concentrations wereprepared. Native (acid solubilized) type I collagen prepared from calfskin (Sigma) and hyaluronic acid prepared from bovine vitreous humor(Sigma) were each dissolved in 0.01 N hydrochloric acid (HCl) to achievedesired concentrations. Dissolved collagen was sterilized by exposure tochloroform overnight at 4° C. Three-dimensional engineered ECMs wereprepared similar to those described in Example 13 at a constant collagentype I concentration (2 mg/ml) and hyaluronic acid concentrations ofbetween 0 and 1.0 mg/ml. The polymerization buffer consisted of 10×phosphate buffered saline (PBS) with an ionic strength of 0.14 M and apH of 7.4. All 3D engineered ECMs and tissue constructs were polymerizedin vitro within a humidified environment at 37° C. To determine thecellular signaling capacity of each 3D microenvironment, 3D tissueconstructs were formed by first harvesting NHDFs in complete media andthen adding the cells (5×10⁴ cells/ml) as the last component to thecollagen solutions prior to polymerization. Immediately followingpolymerization complete media was added and the constructs weremaintained in a humidified atmosphere of 5% CO₂ in air at 37° C.

Qualitative and Quantitative Analysis of 3D ECM Microstructure

Two quantitative parameters describing the 3D fibril microstructuralcomposition of the ECM, fibril area fraction (a 2D approximation of 3Dfibril density) and fibril diameter, were determined based upon confocalreflection and scanning electron microscopy (SEM) images. Prior tomicrostructural analysis, engineered 3D ECM constructs were polymerizedwithin four-well Lab-Tek coverglass chambers (Nalge Nunc International,Rochester, N.Y.) and placed within a humidified environment at 37° C.where they were maintained for approximately 15 hours. For measurementsof fibril area fraction, the confocal microscope was used to obtain highresolution, 3D, reflection images of the component collagen fibrilswithin each ECM. Three images (at least 10 μm in thickness) were takenat random locations within specimens representing a given 3D ECMcomposition. The confocal image stacks were then read into Matlab (TheMathworks, Natick, Mass.), and 2D projections of each image were createdand a threshold chosen for binarization. Using a built-in function inMatlab, the area occupied by collagen fibrils (white pixels) wascalculated, converted to μm2 based upon the pixel sizes, and normalizedto the total image area.

Fibril diameter measurements were made by applying Imaris 4.0 (BitplaneInc., Saint Paul, Minn.) to both confocal reflection and SEM images ofengineered ECM constructs. For SEM imaging, engineered ECM constructswere fixed in 3% glutaraldehyde in 0.1M cacodylate at pH 7.4, dehydratedwith ethanol, and critical point dried. Samples were sputter-coated withgold/palladium prior to imaging. Samples were imaged in at leastduplicate with a JEOL (Peabody, Mass.) JSM-840 SEM. From each imageobtained, twenty fibrils were chosen at random (5 fibrils per quadrant).Five lines were drawn perpendicular to the long axis of each fibrilusing the measurement tool in Imaris. The average number of pixelsrepresenting the fibril diameter was then converted into μm based uponthe known pixel size.

Dynamic Mechanical Testing of 3D Engineered ECMs

Mechanical properties of the engineered ECMs were measured using a TAInstruments (New Castle, Del.) AR-2000 rheometer. Soluble ECMpreparations were adjusted to specific polymerization conditions andplaced on the peltier temperature-controlled lower plate at 22° C., andthe 40-mm parallel-plate geometry was lowered to a 1-mm gap. Thetemperature was then raised to 37° C. to initiate polymerization. Thepeltier heated plate required about 1 minute to stabilize at 37° C.Measurements of storage modulus G′ and loss modulus G″ of thepolymerizing material under controlled-strain oscillatory shear weremade every 30 seconds under oscillation at 1 Hz and 0.1% strain for aproscribed time. This strain was sufficiently small to ensure that itdid not affect the kinetics of polymerization. Two hours and thirtyminutes after polymerization, a shear creep test was conducted with ashear stress of 1 Pa for 120 seconds. Creep data was interpreted with astandard four-element Voigt spring dashpot model. Next a frequency sweepof controlled-strain oscillatory shear was made at 0.1% strain, from0.01 to 20 Hz. Following the frequency sweep, a continuous shear stressramp from 0.1 to 10.0 Pa over 2 minutes was applied. Finally, thespecimen was subjected to unconfined compression at a rate of 10 μm/sec.

Qualitative and Quantitative Determination of Cell Proliferation

Quantification of NHDF proliferation and its dependency on the 3D ECMmicroenvironment involved preparing 3D tissue constructs within 24-welltissue-culture plates. For comparison purposes, the proliferativecapacity of NHDF was also determined for an equivalent number of cellsseeded directly onto the surface of tissue culture plastic. Attimepoints representing 24 and 48 hours after construct polymerizationand/or cell seeding, each well and tissue construct was examinedmicroscopically to observe the viability, number, and morphology of thecells. The medium from each well then was replaced with fresh mediumcontaining the metabolic indicator dye alamarBlue (10% v/v; BioSourceInternational, Inc., Camarillo, Calif.). Approximately 18 hours later,dye reduction was monitored spectrofluorometrically using a FluoroCountMicroplate Fluorometer (Packard Instruments, Meriden, Conn.) withexcitation and emission wavelengths of 560 nm and 590 nm, respectively.Background fluorescence measurements were determined from wellscontaining only dye reagent in culture medium. Maximum levels ofrelative fluorescence were determined from alamarBlue solutions thatwere autoclaved to induce complete dye reduction. The mean and thestandard deviation values for all fluorescence measurements werecalculated and subsequently normalized with respect to the backgroundand maximum fluorescence readings.

Time-Lapse Imaging of Cell-ECM Interactions

Tissue constructs representing NHDFs seeded at 5×10⁴ cells/ml within 3Dengineered ECMs with defined microstructural and biochemicalcompositions were evaluated using time-lapse confocal microscopy.Beginning 1 hour after polymerization, 2 to 3 cells were repeatedlymonitored using the confocal microscope in a reflection (back-scatteredlight) mode to obtain image stacks of the individual cell and itssurrounding matrix as described previously (Voytik-Harbin, et al.,Microscopy and Microanalysis, 9:74-85, 2003). Images were collected at30-minute intervals and a z-step of 0.5 mm to minimize exposure of thetissue constructs to radiation from the argon laser.

Determination of Volumetric Strain

Consecutive confocal reflection images representing temporal deformationinduced by a resident cell on its surrounding ECM microstructureprovided the basis for the quantification of local displacements andstrains in 3D. Within each image, subvolumes of 32×32×20 pixels in thex, y, and z directions, respectively, were established. Each subvolumerepresented a group of voxels centered around a given point at whichdisplacement values were sought. Each image subvolume provided a unique3D voxel intensity pattern that allowed correlation pattern matchingbetween consecutive images using an incremental digital volumecorrelation (IDVC) algorithm developed previously by our laboratory(Roeder et al., J. Biomech. Eng. vol. 124, pp. 214-222 (2002)). The IDVCalgorithm provided strain-state data, including principal strains andtheir associated directions, for all grid point locations. Grid pointswere established in 512·512-pixel images that were 32 pixels apart inboth x- and y-directions, with 24-pixel spacing in the z-direction.Principal strains determined for the length (E_(L)), width (E_(W)), andheight (E_(H)) directions were used to calculate volumetric strain(E_(V)) based upon the following formula:

E _(V) =E _(H) +E _(W) +E _(L)+(E _(W) ·E _(H))+(E _(L) ·E _(H))+(E _(L)·E _(W))+(E _(L) ·E _(W) ·E _(H)).

Determination of 3D Cell Morphology

Prior to imaging at either 6 or 12 hours after construct polymerization,tissue constructs were stained with the vital dye Cell Tracker Green(Molecular Probes, Eugene, Oreg.) to facilitate discrimination of thecell from the surrounding collagen ECM. Confocal image stacks were thencollected in a combined reflection-epifluorescence mode fordetermination of cell morphology and fibril microstructuralorganization.

Results

Fibril diameter distribution was measured, as determined from scanningelectron microscopy images, for engineered matrices prepared from type Icollagen in the presence of varied amounts of hyaluronic acid. Over therange of hyaluronic acid concentrations tested, no significantdifference was observed in mean fibril diameter. Mean fibril diametermeasurements were 80.8±18.3 μm, 72.2±13.0 μm, and 72.1±11.8 μm(±standard deviation) for engineered matrices prepared from 2 mg/ml typeI collagen containing 0, 0.5 mg/ml, and 1.0 mg/ml hyaluronic acid,respectively. Interestingly, it did appear that the variation (standarddeviation) of fibril diameter measurement decreased with increasinghyaluronic acid content. No observable or quantitative differences infibril area fraction measurements were determined for the engineeredmatrices prepared with and without hyaluronic acid.

While hyaluronic acid did not dramatically effect the fibrilmicrostructure of engineered matrices, the polymerization rate was foundto decrease with increasing hyaluronic acid content as indicated by adecreased slope of the G′ versus time plot. Furthermore, as thehyaluronic acid content increased, the engineered matrices showed anincrease in compliance and an increase in their compressive stiffness,respectively. These results demonstrate that the 3D fibrilmicrostructure as well as the viscous fluid component provide criticaldeterminants of the overall mechanical properties of the engineeredECMs.

Studies comparing the cell response to 3D ECM microenvironments preparedwith various hyaluronic acid content have shown no significantdifference in the proliferation properties of neonatal human dermalfibroblasts. However, analysis of cell morphology and matrix contraction(remodeling) by cells indicate that hyaluronic acid alters the mechanicsof cell-ECM interactions. Analyses of the magnitude and spatialdistribution of local, 3D strain induced by a resident cell within anengineered matrix microenvironment revealed that the addition ofhyaluronic acid reduces the ability of fibroblasts to effectivelycontract and induce alignment of surrounding collagen fibrils. In otherwords, the extent of fibril deformation and realignment (remodeling) bycells is decreased and more uniformly distributed around the cell in thepresence of increased concentrations of hyaluronic acid.

Results of these studies show that while the addition of hyaluronic aciddoes not dramatically affect the 3D fibril microstructure of theresultant engineered matrices, it does affect the mechanical properties,likely by changing the properties of the viscous fluid component.Furthermore, systematic variation of the viscous fluid component as aspecific design criteria for 3D engineered matrices does affect themechanisms by which resident cells mechanically manipulate (contract) orremodel their ECM microenvironment.

1. A composition for supporting stem cells, said composition comprisinga synthetic three dimensional matrix comprised of collagen fibrils; anda population of stem cells entrapped within the three dimensionalmatrix.
 2. The composition of claim 1 wherein the synthetic threedimensional matrix has a fibril area fraction of about 8% to about 18%.3. The composition of claim 1 wherein the synthetic three dimensionalmatrix has a fibril area fraction of about 19% to about 26%.
 4. Thecomposition of claim 1 wherein the synthetic three dimensional matrixhas a elastic or linear modulus of about 0.5 kPa to about 24.0 kPa. 5.The composition of claim 1 wherein the synthetic three dimensionalmatrix has a elastic or linear modulus of about 25.6 to about 40.2 kPa.6. The composition of claim 1 wherein said matrix is synthesized bypolymerizing a solubilized collagen composition, said solubilizedcollagen composition comprising said stem cells at a cell density ofabout 1×10³ to about 10⁵ cells per milliliter.
 7. The composition ofclaim 1 wherein the cells are added at a density of about 10 to about10³ cells per milliliter.
 8. The composition of claim 1 wherein thefibril area fraction of the three dimensional matrix is about 7.7% toabout 25%, and the three dimensional matrix further comprisesexogenously added glucose and calcium chloride.
 9. The composition ofclaim 8 wherein glucose concentration is about 5.55 mM to about 30 mMand the CaCl₂ concentration is about 0.2267 mM to about 1.8 mM.
 10. Thecomposition of claim 8 wherein the three dimensional matrix is a threedimensional purified collagen matrix.
 11. An improved method forculturing stem cells said method comprising providing a solubilizedcollagen composition; adding stem cells at a cell density within twoorders of magnitude of the minimum cell number required to maintain cellviability; polymerizing the collagen composition to form a 3D matrixcomprising cells entrapped within a collagen fibril network; andproviding conditions conducive to cell growth.
 12. The method of claim11 wherein the cells are added at a density of less than 5×10⁴ cells permilliliter.
 13. The method of claim 12 wherein the cells are added at adensity of about 10 to about 10³ per milliliter.
 14. The method of claim11 wherein the step of providing conditions conducive to cell growthcomprises the step of implanting the 3D matrix into a host species. 15.The method of claim 11 wherein the step of providing conditionsconducive to cell growth comprises the step of culturing the cells invitro.
 16. The method of claim 11 wherein the solubilized collagencomposition comprises collagen derived from a naturally occurringextracellular matrix.
 17. The method of claim 11 wherein the solubilizedcollagen composition is prepared from a composition consistingessentially of purified type I collagen.
 18. The method of claim 11wherein the solubilized collagen composition is prepared from acomposition consisting essentially of purified type I and type IIIcollagen.
 19. The method of claim 11 further comprising the step ofadding glucose and calcium chloride to the solubilized collagencomposition prior to the polymerization step.
 20. An improved tissuegraft construct comprising a synthetic three dimensional matrixcomprising collagen fibrils; and a population of stem cells entrappedwithin the three dimensional matrix, wherein said synthetic threedimensional matrix has a fibril area fraction of about 7.7% to about25%, has a elastic or linear modulus of about 0.5 to about 40 kPa andfurther comprises exogenously added source of glucose and calciumchloride.
 21. The graft construct of claim 20 wherein said syntheticthree dimensional matrix is formed by contacting a source of collagenwith an acid selected from the group consisting of hydrochloric acid,acetic acid, formic acid, citric acid, lactic acid, sulfuric acid,ethanoic acid, carbonic acid, nitric acid and phosphoric acid to form anacid treated collagen composition; solubilizing the acid treatedcollagen to form a solubilized collagen composition; and polymerizingthe solubilized collagen composition to form said three dimensionalmatrix comprising collagen fibrils surrounded by an interfibrillarfluid, wherein said population of stem cells are added to thesolubilized composition at a density of less than 10⁵ cells permilliliter.
 22. The graft construct of claim 20 wherein the source ofcollagen comprises a composition formed from purified type I collagen.23. The graft construct of claim 22 wherein the source of collagencomprises composition formed from purified type I and type III collagen.24. The graft construct of claim 20 wherein the three dimensional matrixis a three dimensional purified collagen matrix.
 25. A method ofenhancing the repair of tissues in a warm blooded vertebrate, saidmethod comprising implanting or injecting the construct of claim 20 intosaid vertebrate at a site in need of repair.
 26. A method of isolatingclonal populations of individual stem cells, said method comprising thesteps of contacting a 3D matrix with a low density of stem cells whereinsaid collagen matrix is formed by contacting a source of collagen withhydrochloric acid to prepare a solubilized collagen composition;polymerizing the solubilized collagen composition using a final collagenconcentration of 1.0 to 3.0 mg/ml, at a pH of about 6.5 to about 7.0,wherein the initial seed population of stem cells ranges from about 10to about 10³ cells per milliliter; culturing said seeded stem cells andisolating individual population of stem cells.